Add `spatial_extent` input to `sentinel1_sar_interferogram`

[mattiacallegari]

Add support for spatial_extent input parameter to sentinel1_sar_interferogram, matching the existing functionality in sentinel1_sar_coherence

[EmileSonneveld]

Related: #78

[clausmichele]

If we allow the spatial_extent parameter, the inSAR pairs must also be optional, since you can't know in advance which acquisitions will be selected. My proposal is to introduce also the temporal_baseline parameter as in the coherence process and generate the interferograms with subsequent pairs in the same way the coherence does. It will be more tricky to document.