API: `rotate_by` has no business being an attribute

[neutrinoceros]

I think we initially thought of this as a way to retain information about mutations, but (Gas)Field is now immutable.
Of course we need to preserve support for rotation, but I think it should be an ordinary field operation rather than state which complicates all other operations' APIs.

[neutrinoceros]

(a similar argument could be made about the "operation" attribute, but this one might require more discussion)

[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