Use kwargs instead of strict dict

[cgiovanetti]

Minimal changes to allow the user to specify everything in Model.__init__ as kwargs, rather than forcing a dict. Dict is still supported as long as the dictionary is unpacked as it is passed in (e.g. for a dictionary specs, the user can use model = Model(**specs)).

Also added __call__ to main.Model, which just calls run_cosmology.

2/3 example notebooks updated. ABCMB_Basics.ipynb needs an overhaul in light of new output structure, so I only just began to update it.

[cgiovanetti]

No meaningful difference in compile time on A100