Improve MPI encapsulation in par_vec module

Description

Only use HAVE_MPI around the MPI calls. This reduces the need to use HAVE_MPI in other parts of the code. This makes the code better readable.

changed milestone to %11.0

added Refactoring label

Codecov Report

Merging #1126 (de32d25e) into develop (0a0c868a) will decrease coverage by 0.06%. The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #1126      +/-   ##
===========================================
- Coverage    70.11%   70.04%   -0.07%     
===========================================
  Files          515      515              
  Lines       102629   102724      +95     
===========================================
+ Hits         71957    71958       +1     
- Misses       30672    30766      +94     

Impacted Files	Coverage Δ
src/grid/cube_function_inc.F90	82.75% <ø> (ø)
src/grid/double_grid.F90	81.25% <ø> (+0.23%)
src/grid/io_function_inc.F90	60.90% <ø> (-0.30%)
src/grid/mesh.F90	73.00% <ø> (ø)
src/grid/mesh_cube_parallel_map.F90	55.88% <ø> (ø)
src/grid/mesh_interpolation_inc.F90	97.39% <ø> (ø)
src/grid/multigrid.F90	93.12% <ø> (ø)
src/grid/nl_operator.F90	66.07% <ø> (+0.33%)
src/grid/par_vec.F90	67.24% <ø> (-24.46%)
src/grid/par_vec_inc.F90	50.00% <ø> (ø)
... and 7 more

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assigned to @micael.oliveira

approved this merge request

mentioned in commit c0033538

merged