Fix shapefile support

[j08lue]

Currently, shapefiles and GeoJSON files are treated very differently: Geometries from GeoJSON files are read and used like in sentinelsat. From shapefiles, I extract only the extent.

On a more general note, the shapefile support could be considered beyond the scope of the script - since it is not in sentinelsat either. But at least for the project I want to use this script for, I need to support shapefiles. And they are (sadly) still quite common.

So my plan is at least to have the script use the actual geometry from the shapefile and maybe merge the geometry file input to one field and have the script determine which file format it got.