add `prep_nisar` to load NISAR GUNW products

[mirzaees]

Description of proposed changes

Add prep_nisar.py script to read GUNW products. The inputs are a list of nisar*.h5 data cubes and the DEM.

Future to do list:

• The water mask in the test products is all zero. To avoid masking the data, it was manually set to one. We need to remove manual part after correctly adding water mask to input data.
• Tropospheric correction of Mintpy using pyaps does not perform well because of UTM coordinate system. We might read the correction layers from input data instead in the future. But for now, the height_correlation option works.
• The x/y coordinates of data cube in radarGrid require to add a margin to cover the x/y coordinates of GUNW. For now the common bounding box to read all data is the area covered by both radarGrid and GUNW grid, meaning we cut the outer edges if needed.
• The auto path for the input products is set to "../interferograms/*.h5". It might change based on where the GUNW products are located after processing with isce3

[mirzaees]

Thank you @yunjunz for reviewing I do not have an open interferogram stack but I have tested this, and confirm it is working. So let's merge it if it is fine with you.