Complex-valued and single-precision data output in ExodusII/Nemesis format

This commit implements correct complex-valued output to ExodusII and Nemesis formats. In addition, a flag is added to the ExodusII_IO constructor to force single-precision output.

Both features have been tested with nodal data in ExodusII format with the reduced_basis_ex7 example (Nemesis and elemental data not tested yet).