Forward FFT for the non-periodic x case.
Transposes x and y so that the non-periodic direction sits in dim2,
where it keeps its full mode range, then transforms the (ny, nx, nz)
buffer with the swapped plan set up in init.
Nodes of different colours represent the following:
Solid arrows point from a procedure to one which it calls. Dashed
arrows point from an interface to procedures which implement that interface.
This could include the module procedures in a generic interface or the
implementation in a submodule of an interface in a parent module.
Where possible, edges connecting nodes are
given different colours to make them easier to distinguish in
large graphs.