update gmtsar support for the new ifg stack structure

[yunjunz]

Description of proposed changes

This PR updates support for GMTSAR interferogram stack as prepared for the EarthScope short course 2024 by Xiaohua Xu.

• utils.readfile: add GMTSAR_SENSOR_ID2NAME, and call it in _attribute_gmtsar2roipac() to get PLATFORM

• utils.ptime: add yyyyddd2yyyymmdd() to convert from GMTSAR date format into YYYYMMDD format

• prep_gmtsar: update to the new structure as prepared by Xiaohua Xu for the 2024 EarthScope short course. Major changes include: 1) use supermaster.PRM for common metadata extraction and 2) use baseline_table.dat to read the bperp TS directly.

• docs/dir_structure: update to the latest structure.

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