Implement model.dat format versioning

[certik]

• Add a version into model.dat
• Increment the version with every change in the format (in both the Python writer and Fortran reader)
• In the reader, check the version, if it does not agree, refuse to load it.

This will allow to keep changing the format without breaking things. If an older version of fastGPT is used on a newer file, it will say "Incompatible model.dat version, use the create_model.py script to generate a compatible version."