compiler: Support for C-level MPI_Allreduce

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 93.80952% with 13 lines in your changes are missing coverage. Please review.

Project coverage is 86.86%. Comparing base (602d5e6) to head (35aea7c).

Files	Patch %	Lines
devito/core/gpu.py	33.33%	4 Missing :warning:
devito/mpi/reduction_scheme.py	87.50%	3 Missing :warning:
devito/operator/operator.py	62.50%	2 Missing and 1 partial :warning:
devito/ir/clusters/algorithms.py	95.34%	2 Missing :warning:
devito/ir/support/basic.py	66.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2344 +/- ## ========================================== + Coverage 86.81% 86.86% +0.04% ========================================== Files 233 234 +1 Lines 43759 43886 +127 Branches 8076 8102 +26 ========================================== + Hits 37990 38122 +132 + Misses 5061 5059 -2 + Partials 708 705 -3 ```

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