First pass at spectral coarse solver.

This is a full `draft' of the spectral coarse solver using fftw's mpi
transpose to do the heavy lifting.  Moved the shape_x, start_x, end_x
parameters from genmesh to mesh_mod.F90 so they can be accessed by
the coarse solver, which represents most of the collateral changes.

The code is currently being run by the result is zero'ed out at the
end, which has the effect of turning off the coarse solver while still
turning up run-time bugs.  The bracketing scatter/gathers and spectral
transforms are actually unitary, which is good.  However, when the
u = 0._dp line is removed, the subsequent gmres call throws an
arithmetic fit, indicating that there is a problem with the application
of the Green's function.  I think the mapping from index to wave-vector
is wrong, and will look into that next.