Sort out DEM resampling for InSAR

[talogan]

Possibly out of scope for this PR, but I notice the DEM geotiff comes out with a pixel size of 80m/40m for 20x4/10x2 looks, even though we originally downsample the input DEM to 160m/80m. I looks like that's because we set oversampling to 2 when calling GC_map_mod and gc_map, which resamples the DEM back to 80m/40m:
https://github.com/ASFHyP3/hyp3-lib/blob/develop/hyp3lib/SLC_copy_S1_fullSW.py#L63

That seems unproductive? Should we resample the input DEM to 80m/40m at the start and avoid possibly throwing away resolution?

Originally posted by @asjohnston-asf in #247 (comment)