[ML] Use vtk's native methods for getting ids.

In the conversion of a vtkUnstructuredGrid replace
the 5*(nr of elements) GetCell() calls with vtk's
provided function.

The 5*(nr of elements) GetCell() calls make around
50% of total instruction spent in
convertUnstructuredGrid().

For current groundwater flow simulation without the
linear solver call it makes around 1/6 of total
instructions.