Refactor: MPI Distribution and Precision

This MR splits off changes originally in MR !2605 (merged), on small changes/refactoring/extensions of MPI routines.

Changes include:

• Extend comm_allreduce and mesh wrappers of reduce to support int64
• Introduce is_root type-bound procedure for mpi_type
• Introduce a routine to compute indexing offsets (displacements) for use with gatherv/scatterv

Unrelated:

• Doxygen and formatting
• LLM sphere radius for a single-atom system is turned into a constant - previously it was hard-coded in two places