save pruned ast if path provided

[ThrudPrimrose]

Generating SDFGs involve saving the pruned ast
I added these lines:
import os
FORTRAN_AST_PATH = os.getenv("FORTRAN_AST_PATH")
if FORTRAN_AST_PATH is not None:
with open(FORTRAN_AST_PATH, 'w') as f:
f.write(ast.tofortran())

(my editor automatically removes empty lines that only have whitespace)

It can streamline the process of parsing SDFGs

[ThrudPrimrose]

If we want to comment out a function, we need to save this and call it again.
So saving this always if a path is provided is a good idea IMHO

[pratyai]

Generally, I'd rather advice against looking for environment variable in the middle of a library code, and insist on adding yet another Optional[Path] flag to the ParserConfig instead. Or even better, I'd suggest taking the FParser processing part out to the CLI and let it decide if it wants to save the file or not.

However, create_sdfg_from_fortran_file_with_options() is sadly already in quite an "anything goes" state. So, I'm okay with this too if Lex accepts it.