Inversion now takes ice divides into account

[fmaussion]

This PR is a bit larger than I wish it would be, and it brakes backwards compatibility. Non-exhaustive list of changes:

• the inversion formula takes ice divides into account (this changes the thickness estimation locally)
• the filtering that occurred during the inversion is now happening in a separate task. It has been improved to conserve volume entirely. The implications of this change aren't fully explored
• had to make some change to remove rasterio warnings and other issues related to Get rid of intermediate DEM files #216 . The differences between rasterio 1.0 and previous is a pain. (Travis still uses 0.36 and so did I).
• docs improvements

To get something close to backwards compatibility one should set cfg.PARAMS['invert_with_rectangular'] = False in the script. However I would recommend OGGM users NOT to update OGGM until the rest of the workflow can cope with rectangular beds.

[fmaussion]

@anoukvlug @bearecinos fyi see message above

[bearecinos]

sorry I come back to this until now. several questions in the inversion.py :

1. to make sure that I understand. The last pixels of tidewater out_thick[-5:] are kept as they were without filtering and then restore after filtering?
2. After running the vol_inversion() task I need to run this new filtering? Before my calving loop starts for example.
3. Is there any update on using a different bed shape for the las out_thick[-5:] in TW? perhaps could be added as a filtering for tidewater

[fmaussion]

For your purposes I would try to avoid filtering altogether. What would e good for you though is to force the last pixels of the inversion to be rectangular. This should happen in the prepare_for_inversion task