Latex ipython template: draw a vertical line left of ipython code

[timhoffm]

This PR adds vertical lines to the left of code blocks in Latex output. The lines are colored in the respective input/output color.

IMHO this greatly improves readability as it's easier to distinguish between code and text and between input and output. Also it helps to see which code was originally grouped in the cells.

See the following example:

[blink1073]

Thanks, @timhoffm! @tgeorgeux, does this look good to you?

[Carreau]

I like it.

[tgeorgeux]

Looks great to me. Thank you for doing this @timhoffm !

[t-makaro]

Can we reconsider? I don't like that this occurs at the document_contents.tplx level. I have a template that's designed to mimic the notebook more closely here.

The extra line is super out of place in my templates, and I'll have to overwrite this change.

[t-makaro]

I also really don't like that the line extends to the extra newline, and that the line is to the left of the prompt and not between the prompt and the code (but I understand that with the implementation that that's not easy).

Since this is a stylistic thing, I think it should occur at the level of style_type.tplx, and not document_contents.tplx if at all.

[t-makaro]

I also just realised that this change will mess with people who use nbconvert to produce reports by hiding the code from the converted document since the lines will again be out of place.

[mgeier]

Just for reference, here's how I'm planning to re-design the LaTeX representation of code cells in nbsphinx: spatialaudio/nbsphinx#218.

[MSeal]

I'm going to remove the 5.5 milestone on this PR, as we're about to release and there hasn't been a resolution to concerns raised sufficient to warrant merging, changing, or closing the PR imho.

[mgeier]

There is an IMHO more promising approach in #992.

[timhoffm]

Closing in favor of #992.