Add NISAR loading and auxiliary stack support (#1487)

* Add ionosphere stack file to prep_aria arguments

* bug fix for loading ARIA generated ionosphere stack

* Add NISAR auxiliary stacks and improve prep_nisar grid handling

* Add early validation for NISAR inputs in load_data

* Refactor prep_nisar stack dispatch and interpolation setup

* Remove duplicate return statement in prep_nisar.py

Removed redundant return statement from the function.

* populate bperp from NISAR perpendicularBaseline

* import osr explicitly for raster EPSG parsing

* Stop globbing DEM input for NISAR load_data

* Use native GUNW mask as primary mask

* support frequency selection and optional layers

* fix Codacy issues in prep_nisar

* fix remaining Codacy issues

* fix prep_nisar docstring style

* use common NISAR mask across all GUNW inputs. Read mask bits

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Co-authored-by: Zhang Yunjun <yunjunz@outlook.com>