Don't use the 74-78 columns for 5th element CHEMKIN syntax.

Although the CHEMKIN manual specifies that one may put
the 5th element atom count in columns 74-78 of row 1,
in practice nobody does this, and Cantera cannot read it
if we do, so this commit stops us from doing this.
Instead if there are 5 or more elements present, we use
the extended syntax (that we were using for 6+ elements).

For more details see discussion at 
Cantera/cantera#656

rmgpy/chemkin.pyx

@@ -1535,8 +1535,11 @@ def write_thermo_entry(species, element_counts=None, verbose=True):
 
     # Line 1
     string += '{0:<16}        '.format(get_species_identifier(species))
-    if len(element_counts) <= 5:
+    if len(element_counts) <= 4:
         # Use the original Chemkin syntax for the element counts
+        # Up to the first 4 elements only
+        # (don't use the space in columns 74-78 for the 5th element
+        #  because nobody else does and Cantera can't read it)
         elements = []
         for key, count in element_counts.items():
             if isinstance(key, tuple):
@@ -1545,23 +1548,22 @@ def write_thermo_entry(species, element_counts=None, verbose=True):
             else:
                 chemkin_name = key
             elements.append('{0!s:<2}{1:>3d}'.format(chemkin_name, count))
-        elements.extend(['     '] * (5 - len(element_counts)))
+        elements.extend(['     '] * (4 - len(element_counts)))
         string += ''.join(elements[0:4])
-        string += 'G{0:>10.3f}{1:>10.3f}{2:>8.2f}{3} 1'.format(
+        string += 'G{0:>10.3f}{1:>10.3f}{2:>8.2f}      1'.format(
             thermo.polynomials[0].Tmin.value_si,
             thermo.polynomials[1].Tmax.value_si,
             thermo.polynomials[0].Tmax.value_si,
-            elements[4]
         )
     else:
+        # Use the new-style Chemkin syntax for the element counts
+        # This will only be recognized by Chemkin 4 or later
         string += '                    G{0:>10.3f}{1:>10.3f}{2:>8.2f}      1'.format(
             thermo.polynomials[0].Tmin.value_si,
             thermo.polynomials[1].Tmax.value_si,
             thermo.polynomials[0].Tmax.value_si,
         )
         string += '&\n'
-        # Use the new-style Chemkin syntax for the element counts
-        # This will only be recognized by Chemkin 4 or later
         elements = []
         for key, count in element_counts.items():
             if isinstance(key, tuple):

rmgpy/chemkinTest.py

@@ -458,7 +458,8 @@ def setUp(self):
 -5.99027000E-08 2.38945000E-11-1.12576000E+04 3.56130000E+00                   4
 """
 
-        self.entry2 = """CH3NO2X                 C   1H   3N   1O   2G   300.000  3000.000  650.73X   1 1
+        self.entry2 = """CH3NO2X                                     G   300.000  3000.000  650.73      1&
+C 1 H 3 N 1 O 2 X 1
 -3.07954000E-01 2.45269000E-02-1.24130000E-05 3.07724000E-09-3.01467000E-13    2
 -1.06930000E+04 2.26280000E+01 4.03055000E+00-2.14171000E-03 4.90611000E-05    3
 -5.99027000E-08 2.38945000E-11-1.12576000E+04 3.56130000E+00                   4