plate motion package

/docs   -  documentatios, notebooks
/src/{pkg_name}
    # low-level scripts
    euler_pole.py              # object definition
    plate_motion_model/pmm.py  # PMM info
    # high-level
    plate_motion.py            # calculate PMM in ENU
    plot/utils.py              # funcs to read and plot PMM
    /data
        /GSRM
        /MORVEL56
/tests
    ...

For the current mintpy PR:

• cli/plate_moton: ITRF2014_PMM_PLATES =

• euler_pole += plot, pmm

• All upper case for plate abbreviations

• move --plate to --om-cart from mintpy.cli.plate_motion to mintpy.plate_motion