`prep_gamma`: support geocoded Gamma products

[yunjunz]

Description of proposed changes

• prep_gamma: add --dem option to specify the corresponding DEM file, in order to:
  1. determine the dataset coordinate type
  2. extract the corresponding geo-coord specific metadata

Reminders

• [x] Pass Pre-commit check (green)
• [x] Pass Codacy code review (green)
• [x] Pass Circle CI unit test (the circleci integration test is failing due to the known gdal issue https://github.com/insarlab/MintPy/issues/1220)
• [x] Pass local integration test
• [x] Make sure that your code follows our style. Use the other functions/files as a basis.
• [x] If adding new functionality, add a detailed description to the documentation and/or an example.