New MB model calibration architecture

[fmaussion]

This opens the door to add @lilianschuster 's MB sandbox tools.

Currently only with Monthly model

I added a flexible MB calibration scheme, including the infamous three-step calibration

[fmaussion]

This has become much larger than anticipated, but it feels good. The decommissioning of mu* is now!

[fmaussion]

Some todos:

• override_missing kwarg in the new calibration could accomodate with a dict or a dataframe of values to allow for arbitrary datasets
• docstrings of the most important functions: calibration, MB models, run_
• thinking about the naming of functions and parameters really hard as we wont change them back afterwards
• the merged glacier funcs need multiple flowline parameters - looking into the future someone else might need this. Do we want to try to implement? there are already some ideas here: https://github.com/fmaussion/oggm/blob/0a5b8544b9b89ce8eda43fd0fbe12f36538daa1d/oggm/core/massbalance.py#L591-L598