Calculation of bandwidth broke in commit fb97bcd0

Check of MpiCommunicator::IsInitialized is necessary because
non-distributed benchmarks like tnl-benchmark-blas do not initialize
the communicator.

It does not have any effect on recent builds and we explicitly set
-march=native -mtune=native later in the file.

Distributed linear solvers

See merge request mmg/tnl-dev!8