Multiband rasters as input: improve band selection

[remtav]

Based on PR #309, a band selection should be possible for multiband rasters. An easy implementation would be to expect indexes instead of band names for the dataset.bands parameter. Rasterio can use this parameter directly to choose relevant bands. The user would be reponsible of knowing the band corresponding to each index. We'd add this information in our documentation.

This means we'd remove the limit BGR_to_RGB() function. Also the band selection and ordering would happen in the AOI object construction, not as a transform as with BGR_to_RGB().