Encapsulate MPI

[stigrj]

Has been tested to work for:

• Serial client -- serial MRCPP (works)
• Serial client -- parallel MRCPP (works as one would expect, client can use MRCPP as serial lib and no MPI parallelization is gained)
• Parallel client -- serial MRCPP (works as long as there's no reference in client to MRCPPs parallel API, the client's parallelization will be completely independent of MRCPP)
• Parallel client -- parallel MRCPP (works, but need to juggle client's HAVE_MPI vs MRCPP_HAS_MPI)

[stigrj]

This means that a fully parallel (--omp --mpi) MRCPP can now be used in clients with any level of parallelization; serial, omp, mpi or hybrid (as long as it's correctly implemented in the client, ofc)

[codecov[bot]]

Codecov Report

Merging #151 into master will not change coverage. The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #151   +/-   ##
=======================================
  Coverage   67.11%   67.11%           
=======================================
  Files         172      172           
  Lines       12153    12153           
=======================================
  Hits         8156     8156           
  Misses       3997     3997           

Impacted Files	Coverage Δ
src/trees/SerialTree.cpp	98.27% <ø> (ø)
src/utils/Printer.cpp	3.64% <ø> (ø)
src/utils/mpi_utils.cpp	0.00% <0.00%> (ø)

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[MinazoBot]

	2 Warnings
:warning:	There are code changes, but no corresponding tests. Please include tests if this PR introduces any modifications in behavior.
:warning:	Consider adding supporting documentation to this change. Documentation sources can be found in the doc directory.

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