Auxiliary functionality for merging orbital CUBEs

[bskinn]

Some tools (e.g., orca_plot) appear to only generate single-orbital CUBEs. Potentially handy to enable merging into a single file.

Would have to check for the indices, to ensure no collisions, and probably also consistency-check everything in the header (origin, axes, units, number/ID/position of atoms). So, not trivial, but still possibly worthwhile.

Would probably also need functionality for changing the comment(s) of the merged CUBE, since the data will contain more than just what was in one of the source CUBEs.

[bskinn]

Origin and voxel axes MUST match, for this utility. CUBE file interpolation is a different matter, #64. Could eventually be integrated with this, but not on initial implementation

Definitely want to implement consistency checks for geometry, (other things?), in cases where it's desired to wrap datasets from different systems into a single CUBE. So, by default run the checks and error (for subsequent exit if cmdline) if fail, but provide options to ignore individual checks (or all checks) to allow user override.