WIP rdr2geo + geo2rdr unit tests

[LiangJYu]

This PR is based the rdr2geo and geo2rdr unit tests @hfattahi created for the now deprecated Cython ISCE3. All test inputs are generated by the test. The objective for the this PR is have the 4 following tests:

1. rdr2geo with satellite orbit over equator and zero DEM with zero DEM
2. rdr2geo with satellite orbit over constant 45° lat with zero DEM
3. rdr2geo with satellite orbit over equator and zero DEM with zero DEM
4. rdr2geo with satellite orbit over constant 45° lat with zero DEM

TBD:

• Decide whether to use existing SimpleNamespace to mimic RunConfig shortcut or introduce test yamls and SAFE zip for comprehensiveness. Using SimpleNamespace.

To do:

• debug so rdr2geo test passes
• implement both equator and 45° lat geo2rdr tests
• more documentation and comments

[LiangJYu]

The rdr2geo test is currently failing because the DEM interpolator is returning the wrong height. The indices computed by DEM interpolator do not fall within the synthetic DEM so the default value (DEM average) is returned for all points in the radar grid. I think a couple things need addressing:

• a DEM that fits the radar grid. I'm not sure how to do this.
• ability to change DEM interp method to nearest neighbor so the DEM can remain small. This I can fix.

Since the Cython rdr2geo test tests point by point, it avoids this problem by changing the DEM interpolator default value to expected height per point.