polarization string index in .grd filename in get_polsar_stack function

[madelinedm]

function 'get_polsar_stack' in polsar.py pulls the polarization string from the filename which is inconsistent with at least some of the UAVSAR .grd file formats
When reading the *.grd. files from this scene, https://uavsar.jpl.nasa.gov/cgi-bin/product.pl?jobName=evergl_15704_09044_000_090616_L090_CX_02#data

I needed to modify line 63 in polsar.py to be:

        name = basename(f).split('_')[-4][4:]

rather than
name = basename(f).split('_')[-3][4:]