Bugfix: Detect and ignore NaNs in per-pixel dem error calculation

[adeane-ga]

_This is a small PR for some additions to the dem_error.py file, we should create a new PR for some proper testing of the DEM error corrections._

The per-pixel DEM error estimation loop would fail because the loop would contain NaNs from out of frame regions

• added a for loop skip in dem error estimation for pixels with NaNs

The baseline metadata in the IFG GeoTIFFs were not being updated because of the use of np.mean in the presence of NaNs

• converted 2x np.mean() functions to np.nanmean()

I also did some code tidying