Add band selection to So2Sat Dataset and adopt plot method

[nilsleh]

This PR addresses the last comment by Adam in issue #289 about adding band selection to the So2Sat Dataset. Changes I made:

• add band selection as a sequence and select desired bands in getitem
• adopt the plot method to look for RGB indices and only plot then
• since the dataset takes a specific split as an argument, I thought it would make sense that the integrity check also only checks the specific selected split, so that not all splits need to have been downloaded

The remaining question I have is whether there is a standard naming convention for the S1 bands, described in the dataset repo. I couldn't find one and therefore just prepended "S1".

Plot examples:

[calebrob6]

The remaining question I have is whether there is a standard naming convention for the S1 bands, described in the dataset repo. I couldn't find one and therefore just prepended "S1".

This seems fine to me!

[adamjstewart]

I'm looking back at this and I'm not sure if I like this solution anymore. I think it makes sense to allow uncurated raster datasets like Landsat8 and Sentinel2 to support band selection, but I'm not sure if it makes sense to allow this level of customization for curated benchmark datasets like So2Sat. Is anyone really going to want to evaluate the performance of their model on So2Sat Sentinel-2 bands only but with a subset of the bands, or the bands in a different order? I think it would make more sense to get rid of the bands parameter and instead have a band_set parameter that takes in values like "all", "s1", "s2", "rgb", etc. What do you think? This level of customization still allows us to use So2Sat to create pre-trained models for only Sentinel-1 or Sentinel-2 data (although not with all s2 bands).

P.S. The band names chosen in this PR don't match the band names in our Sentinel2 class, they probably should.