save_roipac: support non-velocity dataset from the velocity file

[bjmarfito]

Description of proposed changes

• Ability to extract a specific dataset from a velocity file and save it in ROIPAC format.
• Removed behavior to make datasets into displacement values with the exception of the velocity dataset

Reminders

• Pass Codacy code review (green)
• Pass Pre-commit check (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.

[yunjunz]

Looks all good to me now. Thank you @bjmarfito for contributing!

[bjmarfito]

You're welcome and thank you @yunjunz!