API: most attributes in `GasField` should be optional and/or removed

[neutrinoceros]

the following attributes are only required to enable planets-related methods and should all be optional. I want to give a shot at getting rid of a many of them as possible for nonos 0.20.0, which is supposed to introduce Field as the long term replacement for GasField

snapshot_uid

This is necessarily meaningful: a field could exist outside of a snapshot.
It might be easier to defer to a (possibly missing) snapshot parent

loader

only parts of a Loader are really ever used:

• loader.parameter_file (this should really just be metadata)
• loader.read_planet_data (a PlanetDataReader would suffice)

operation

this represents, however poorly, a history of various transformations
that were applied to the original field to obtain this one.
This cannot possibly represent every possible transformation, like
addition (in which there is more than one 'parent' field)

rotate_by

this is another attempt to preserve transformation history, though this one
only represents a (possibly approximate, and inacurrate in sign) shift applied in
along an azimuthal axis.

Linked PRs

• API: re-implement Field.rotate as Field.azimuthal_rotation, drop Field.rotate_by as an instance attribute #662

[neutrinoceros]

with #669 in, this is now about GasField, which I'm not planning to deprecate in v0.20.0 anymore, so there is much less incentive to treat this issue as urgent