move expression building logic into init/fit methods

[liamholmes31]

What?
It would be more efficient to move logic which builds expressions out of transform and into init/fit methods (depending on whether the transformer fits).

I had a vague memory of having tried this and run into issues, but just did a quick test and these seem to have vanished (if they were ever there!), possibly newer dependency versions have resolved this for us..

Why?
speed up transform methods

How?
may vary slightly from transformer to transformer, but the general idea will be to move the setup of 'transform_expressions' into an attr set during fit/init and just utilised in transform

[liamholmes31]

ah actually, I think the issue might come from trying to dump expressions to json and back - this might need some figuring out

[liamholmes31]

we could add methods separate to fit/init called e.g. 'prepare_transform_expressions', then in transform the expressions attr is checked for and this method is only called if they have not been set - so this would give a user the option of calling this prior to transform in order to speed things up