Add option to input co- and cross-pol rasters manually for RGB

[hjkristenson]

Currently the input is the HyP3 RTC directory, and it finds the VV.tif and VH.tif (or HH/HV) rasters and runs the process. It would be more flexible to give users the options to set the inputs directly if desired. There could be a checkbox to activate those input parameters, or they could just be visible always but populated by default when the input directory is set.