Description of proposed changes

• add.py:
  • check num of files == 2 for TS file only
  • support two TS file with different reference point, different reference date, different date list; to be consistent with diff.py.
• diff.py: _check_reference() -> check_reference() with more comments
• view: expand --math to multi-subplots
• save_gdal: use float32 as default to save disk
• smallbaselineApp: plot geo_avgSpatialCoh.h5 file
• utils.readfile: more relaxed wildcard filename for offset file from ampcor
• utils.ptime.get_date_range(): add 'dstep' option for coarser list to speedup matrix operation laterward

Reminders

• Pass Codacy code review (green)
• Pass Circle CI / local test (green)
• Make sure that your code follows our style. Use the other functions/files as a basis.
• If modifying functionality, describe changes to function behavior and arguments in a comment below the function declaration.
• If adding new functionality, add a detailed description to the documentation and/or an example.