Finish Fast Poisson Solvers

[noooway]

Prototype several fast solvers for Poisson's equation.
First stage: 2d region, rectangular region, boundary conditions of the first type.

Needed:
FFT (e.g. https://epubs.siam.org/doi/abs/10.1137/1019071)
Cyclic reduction (as above)
FFT+domain decomposition ( http://num-meth.srcc.msu.ru/zhurnal/tom_2015/pdf/v16r110.pdf )

FFT and Cyclic reduction are in progress;