Use consistently count and indexes for band listing and counting (removing nbands and bands)

[rhugonnet]

To be fully consistent with rasterio.

Resolves #280

[adehecq]

Hopefully it will be much clearer now!
But what I don't get is that basically here you just removed nbands into count and bands into indexes. But I thought we had a different use of count and nbands, depending on what was loaded and what was on disk, no?
In the issue, I suggested using a count_loaded and indexes_loaded but it seems like it was not needed, why?

[rhugonnet]

My bad, I forgot that point! I added them back and added tests for them.

The names count_loaded and indexes_loaded sounded good. But then count and index are "on-disk", and can take a value of None, which is not so practical when manipulating in-memory-only Raster. So instead I went for count_on_disk and indexes_on_disk, while count and indexes are in-memory. Clarified everything in the descriptions.

[adehecq]

My bad, I forgot that point! I added them back and added tests for them.

The names count_loaded and indexes_loaded sounded good. But then count and index are "on-disk", and can take a value of None, which is not so practical when manipulating in-memory-only Raster. So instead I went for count_on_disk and indexes_on_disk, while count and indexes are in-memory. Clarified everything in the descriptions.

Sounds good. Everything seems perfect now, thanks !