When dealing with large number of cores with mumps distributed interface,
if the size of the problem is small, poor performance are obtained with
mumps library. The fact that matrix is presented in distributed format
alleviate some internal optimization (available with centralize matrix
format) and mumps among other has difficulty then to estimate its memory
needs. This lead to underestimate memory evaluation during analyse phase
which stop computation during factorization with a -9 error. Even if
this memory issue is bypassed CPU time increase with number of cores
when more then needed are used.

In this commit a extra parameter ratio_reduce_comm_ is added to
connectMatrix method of xLinearSystemSolverMumpsDistributed class.
It correspond some how to an "ideal" ratio between the number of core to
use for a given problem size "n". mx=ratio_reduce_comm * n is giving a
roughs estimate of the maximum number of cores needed by mumps
computation f...