Refactor P2P and MPI handling #24

Job	Run time
Format / pre-commit	33s
Format / Links	18s
Build Cache / OpenMPI 5	41s
PIP / Install	1m 8s
Domain Decomp / Unit	1m 5s
Integration / Dummy	1m 17s
Adaptivity Serial / Unit & Integration	1m 4s
Coverage / Evaluate	1m 59s
User-Side / Unit	1m 56s
Adaptivity Parallel / Integration	1m 10s
Adaptivity Parallel / Unit	1m 6s
	12m 17s

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