core: Work around another boost.mpi bug

[fweik]

Older versions of boost.mpi had a broken implementation of communicator::isend for non-mpi data types. This works around that.

[codecov]

Codecov Report

Merging #2421 into python will increase coverage by <1%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           python   #2421    +/-   ##
=======================================
+ Coverage      72%     72%   +<1%     
=======================================
  Files         397     397            
  Lines       18666   18678    +12     
=======================================
+ Hits        13496   13508    +12     
  Misses       5170    5170

Impacted Files	Coverage Δ
src/core/utils/mpi/sendrecv.hpp	100% <100%> (ø)	⬆️
src/core/unit_tests/sendrecv_test.cpp	100% <100%> (ø)	⬆️

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[mkuron]

I think we might be able to put a fixed version of mpi::communicator::isend into a cpp file, build that to a shared library with -Wl,-z -Wl,interpose and link it into Espresso. Of course, we'd only do that on Linux platforms and only if we are below Boost 1.68.0.

[mkuron]

For reference, this is the patch: boostorg/mpi@e3b0d08. It was first included in Boost 1.67.0.