Inverse plans

[devmotion]

To me it seems a bit unintuitive that AbstractFFT assumes that plans have a field pinv which is populated when calling inv. When computing the inverse via inv I don't expect the provided element to be mutated.

I would suggest to replace plan_inv with inv such that calling inv returns the inverse plan but does not mutate the given plan. Similar to ScaledPlan a wrapper type could be provided that contains a plan and its inverse (e.g. as fields p and pinv). In my opinion, this would make computing and caching of inverse plans more transparent. Additionally, it would allow plans to be immutable and strictly typed (and hence maybe improve performance?).

What do you think?