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.
Can you make the argument `const` to indicate that it is an input argument, and state in the comment where possible values for the argument can be looked up?