gh-35394: Adding a save method to class Standalone/TikzPicture for co…

…mpatibility with sagetex

    
We add a save method to class Standalone/TikzPicture for compatibility
with sagetex. In particular, so that `\sageplot{t}` works when `t` is an
instance of Standalone/TikzPicture classes.

For example the following `file.tex`:

```
\documentclass{article}
\usepackage[margin=2cm]{geometry}
\usepackage{sagetex}
\begin{document}
\begin{sagecommandline}
sage: from sage.misc.latex_standalone import TikzPicture
sage: V = [[1,0,1],[1,0,0],[1,1,0],[0,0,-1],[0,1,0],
....:      [-1,0,0],[0,1,1],[0,0,1],[0,-1,0]]
sage: P = Polyhedron(vertices=V).polar()
sage: s = P.projection().tikz([674,108,-731],112,
output_type='LatexExpr')
sage: t = TikzPicture(s)
\end{sagecommandline}
\begin{center}
    \sageplot{t}
\end{center}
\end{document}
```

will compile fine with the commands:
```
pdflatex file.tex
sage file.sagetex.sage
pdflatex file.tex
```
provided the `sagetex.sty` file is in the same repository.

Since sagetex creates by defaults `eps` and `pdf` images, we added
methods `dvi` and `eps` to the class Standalone following what was done
in the other methods. Also, we added a feature `dvips` to tag the
doctests with this new external feature. Finally, we made a small
improvement to the latex feature by providing more information in the
reason why a latex feature does not work.

- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [x] I have updated the documentation accordingly.
    
URL: #35394
Reported by: Sébastien Labbé
Reviewer(s): Frédéric Chapoton, Sébastien Labbé

src/sage/doctest/external.py

@@ -380,6 +380,7 @@ class AvailableSoftware():
         sage: from sage.doctest.external import external_software, available_software
         sage: external_software
         ['cplex',
+         'dvips',
          'ffmpeg',
          'gurobi',
          'internet',

src/sage/features/latex.py

@@ -50,6 +50,23 @@ def is_functional(self):
             sage: from sage.features.latex import latex
             sage: latex().is_functional()             # optional - latex
             FeatureTestResult('latex', True)
+
+        When the feature is not functional, more information on the reason
+        can be obtained as follows::
+
+            sage: result = latex().is_functional()    # not tested
+            sage: print(result.reason)                # not tested
+            Running latex on a sample file
+            (with command='latex -interaction=nonstopmode tmp_wmpos8ak.tex')
+            returned non-zero exit status='1' with stderr=''
+            and stdout='This is pdfTeX,
+            ...
+            Runaway argument?
+            {document
+            ! File ended while scanning use of \end.
+            ...
+            No pages of output.
+            Transcript written on tmp_wmpos8ak.log.'
         """
         lines = []
         lines.append(r"\documentclass{article}")
@@ -77,8 +94,12 @@ def is_functional(self):
             return FeatureTestResult(self, True)
         else:
             return FeatureTestResult(self, False, reason="Running latex on "
-                                     "a sample file returned non-zero "
-                                     "exit status {}".format(result.returncode))
+                                     "a sample file (with command='{}') returned non-zero "
+                                     "exit status='{}' with stderr='{}' "
+                                     "and stdout='{}'".format(result.args,
+                                                              result.returncode,
+                                                              result.stderr.strip(),
+                                                              result.stdout.strip()))
 
 
 class latex(LaTeX):
@@ -165,6 +186,27 @@ def __init__(self):
         super().__init__("lualatex")
 
 
+class dvips(Executable):
+    r"""
+    A :class:`~sage.features.Feature` describing the presence of ``dvips``
+
+    EXAMPLES::
+
+        sage: from sage.features.latex import dvips
+        sage: dvips().is_present()             # optional - dvips
+        FeatureTestResult('dvips', True)
+    """
+    def __init__(self):
+        r"""
+        TESTS::
+
+            sage: from sage.features.latex import dvips
+            sage: isinstance(dvips(), dvips)
+            True
+        """
+        Executable.__init__(self, "dvips", executable="dvips",
+                            url="https://tug.org/texinfohtml/dvips.html")
+
 class TeXFile(StaticFile):
     r"""
     A :class:`sage.features.Feature` describing the presence of a TeX file
@@ -254,4 +296,5 @@ def all_features():
             pdflatex(),
             xelatex(),
             lualatex(),
+            dvips(),
             LaTeXPackage("tkz-graph")]