public pure function poisson_bc_code(periodic) result(bc_code)
Names the boundary condition case of the FFT Poisson solver, one
digit per direction in x, y, z order: 0 for periodic, 1 otherwise.
The 000 case is periodic everywhere, 110 is non-periodic in x and y.
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.