Added set_matrix_mode to the SLEPcWrappers::TransformationBase class.

this is handy when one needs to specify how the transformed matrices are being stored in the spectral transformations. In particular for a MatrixFree class, the (non-default) ST_MATMODE_SHELL is the simplest way to go.
@bangerth will do. Is giving a HTML page ok for a link to possible values?  http://www.grycap.upv.es/slepc/documentation/current/docs/manualpages/ST/STMatMode.html  Alternatively one could write something like: "The possible values are given by the enumerator STMatMode in the PETSc library"