Fix the mess in attributes nbands/count/bands/indexes

[adehecq]

See the discussion in PR #265 about the craziness in attributes count vs nbands, bands vs indexes.
Here is my suggestion:

I think we should stay consistent with rasterio's naming convention and hence have count and indexes match the definition of rasterio:

• count: band count on disk
• indexes: The 1-based indexes of each band in the dataset

We can have an equivalent of each for bands loaded, i.e. count_loaded and indexes_loaded (not sure the latter is really needed...).
What do you think @erikmannerfelt, @atedstone @rhugonnet ?
Others should be deprecated.

Originally posted by @adehecq in #265 (comment)