Suggestion: remove requirement for specific file extensions

This line requires a specific file extension. I don't think this is necessary. Many programs don't require their files to have specific extensions; they just require files to have specific formats.
https://github.com/ASinanSaglam/PyBioNetGen/blob/e663aabb5d60ac2aef93111c75896701537761b1/bionetgen/modelapi/bngparser.py#L68