Allows selecting output format for PETScSparseMatrix object

* 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.
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