Description of proposed changes

• Ability to extract other dataset from the velocity files in save_gbis.py such as step, and polynomial functions
• This PR is adapted from save_roipac: support non-velocity dataset from the velocity file #874

Reminders

• Pass Pre-commit check (green)
• Pass Codacy code review (green)
• Pass Circle CI test (green)
• Make sure that your code follows our style. Use the other functions/files as a basis.
• If adding new functionality, add a detailed description to the documentation and/or an example.