Forbid certain characters from species identifiers in Chemkin file writer.
Forbid certain characters from species identifiers in Chemkin file writer.

In particular, the "+" and "=" characters are used as delimiters in the
reaction equation, so we don't want to allow them in species identifiers.
As of this commit, the allowed characters are alphanumeric, hyphen,
underscore, comma, parentheses, and asterisk. This should hopefully enable
the parsing of most real-world Chemkin files.

Closes #228.

enochd/RMG-Py,chatelak/RMG-Py,KEHANG/RMG-Py,pierrelb/RMG-Py,nickvandewiele/RMG-Py,comocheng/RMG-Py,nyee/RMG-Py,faribas/RMG-Py,pierrelb/RMG-Py,comocheng/RMG-Py,KEHANG/RMG-Py,faribas/RMG-Py,chatelak/RMG-Py,enochd/RMG-Py,nickvandewiele/RMG-Py,nyee/RMG-Py