Small MPI refactor

[kubagalecki]

• Enabled communication of trivially copyable types
• StaggeredAllGather factored out as repeatable communication pattern

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (682c682) 83.73% compared to head (f6eabc6) 83.63%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #58      +/-   ##
==========================================
- Coverage   83.73%   83.63%   -0.11%     
==========================================
  Files          88       88              
  Lines        4907     4870      -37     
  Branches     2054     2016      -38     
==========================================
- Hits         4109     4073      -36     
- Misses         63       64       +1     
+ Partials      735      733       -2     

Files	Coverage Δ
include/l3ster/dofs/DofIntervals.hpp	88.05% <100.00%> (-1.46%)	⬇️
include/l3ster/util/Algorithm.hpp	84.21% <100.00%> (+6.59%)	⬆️
include/l3ster/util/ArrayOwner.hpp	87.50% <100.00%> (+0.32%)	⬆️
include/l3ster/util/BitsetManip.hpp	96.42% <100.00%> (ø)
include/l3ster/comm/MpiComm.hpp	93.58% <94.44%> (+0.58%)	⬆️
include/l3ster/glob_asm/SparsityGraph.hpp	75.49% <85.71%> (-1.29%)	⬇️
include/l3ster/dofs/NodeCondensation.hpp	91.46% <88.88%> (-1.98%)	⬇️

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