DataSetWriter: Fix overflow encountered at scale

We sometimes encounter an error like the following, but only at large scales (at smaller scales everything works fine):

2025-07-07 20:48:34.155 (1788.135s) [    149BFE244100]
    vtkFidesWriter.cxx:420    ERR| vtkFidesWriter (0x33e0760): Exception encountered when trying to write data:
        [Mon Jul 07 20:48:33 2025] [ADIOS2 EXCEPTION]
            <Core>
            <VariableBase>
            <InitShapeType> : start[0] = 5612190313 > shape[0], = 398441462 in DefineVariable Normals

The fact that it only happens at larger scales hints at some kind of numeric overflow, so this change attempts to prevent overflow by using size_t instead of int for anything we count in the DataSetWriter.

downloadable patch