allow type 1 and 2 plans to be interchangeable to reduce from two to one plan for transform pairs

[ahbarnett]

Feature request: It would be great to allow a t1 plan to be switched to t2 easily, and vice versa.

Motivation: t1 and t2 often come in pairs on the same NU point sets. Apparently making two plans is wasteful of time and RAM. SOme competing NUFFTs allow a single plan.

Execution: This is merely an API/interfacing issue, since the sort ordering and FFT plan should be the same.
Questions if this can be done on CPU and on GPU (should be separate PRs, with doc changes to match).
To discuss...

[ahbarnett]

on GPU Marco thinks would work.
on CPU: I/O arrays are the issue - swaps fw vs fk ?

Impl: finufft.settype ? Annoying language addition.