Added set_matrix_mode to the SLEPcWrappers::TransformationBase class.
Remove a link that is no longer live.
Merge pull request #211 from bangerth/remove-broken-link

Remove a link that is no longer live.
made argument of set_matrix_mode const.
Added a reference to SLEPc with possible values.
Merge pull request #205 from davydden/master

Added set_matrix_mode to the SLEPcWrappers::TransformationBase class.