Test assertion breaking w/ MPICH

[freifrauvonbleifrei]

Since CI with MPICH was introduced, there were a bunch of failing tests due to this check failing:

DisCoTec-gcc-mpich-Debug/tests/test_distributedsparsegrid.cpp(907): error: in "distributedsparsegrid/test_sparseGridAndSubspaceReduce": check subspaceStart[j] == i * nprocs has failed [108071 != 108135]

(code here:

DisCoTec/tests/test_distributedsparsegrid.cpp

Line 912 in ea91e58

BOOST_CHECK_EQUAL(subspaceStart[j], i * nprocs);

)

never for openmpi, often for mpich in both debug and release mode.

???