Handle .vtu files with unsupported offset and cell types

Changes to vtkXMLUnstructuredGridReader restricted the offset and cell DataArray types to int, long, and long long. However, the reader used to support other DataArray types for these arrays and now produces an error when attempting to read a file with this type offset/cell id.

This change checks whether the specified data array type is valid, and if not, converts the data read from the file to an acceptable type (long long to handle potentially large offsets and cell ids). Added test for a non-supported offset and cell array type to verify the conversion works.

(cherry picked from commit 49b763af)