So2Sat: rename bands

[adamjstewart]

Follow-up to #394

I think it's better to ensure that all band names in So2Sat match the band names used in our Sentinel datasets. Since some bands come from Sentinel-1 and some bands come from Sentinel-2, the best way to handle this is to prefix S1_ and S2_ in front of all bands.

Alternative: do Sentinel-1 bands really go by B1, B2, ...? I see a lot of VV/VH/HV/HH online. Someone should implement a Sentinel1 dataset so we can match that. If the bands no longer use B# names, we can drop the prefix from all bands.

@nilsleh