Fix for obtaining BLACS context.

[icosaeder]

Sometimes it is needed to use EigenExa with a dedicated communicator, not just MPI_COMM_WORLD. According to the documentation, the trick is to call eigen_init in each MPI process, supplying either the needed communicator for involved processes, or MPI_COMM_NULL for the rest. However, it did not really work, as the BLACS context was always obtained from the default communicator. This pull request fixes that issue and improves the API so that there is no need to call eigen_init in every process anymore. It is enough to call it in the participating processes only, passing the appropriate communicator.

Should this pull request be accepted, the EigenExa documentation should be amended accordingly. The change should be done in Section 3.2 of EigenExa_Manual. In particular, the second paragraph of Section 3.2 ("Since a different communicator ... including eigen_sx().") should be removed.