clean nfft module code:

- revert support to only double precision NFFT plan
- remove .pxd file for code simplification
merge solver component in nfft module, plus simplify the code base
remove file as part of the code refactoring
finalize code simplification of the solver class
release gil for solver execution to enable openmp speedup
remove deprecated solver extension
move main component import to root module hierarchy
fix typos
update autogen files
adapt tests to new module layout
bump version
reorganize documentation based on module layout change