The classes' print function now accepts a second optional argument of
type PetscViewerFormat. Any valid format is possible, as this will just
be passed on to PETSc. Default value is PETSC_VIEWER_ASCII_MATRIXMARKET.
The exportOperator() function now requires the format as an argument; it
now also lives in the petsc namespace.
The exportOperatorMatrix app allows to select either MATRIXMARKET or
MATLAB format on the command-line.