postProcess with cyclicACMI patches

Summary

Run the oscillatingACMI2D tutorial with some e.g. isoSurface functionObject as ``postProcess``` and it will crash since the pointMesh gets cleared out but the pointField (from volPointInterpolation) does not.

Possible fixes

In polyMesh::readUpdate make sure to update pointMesh (or MeshObject) in general instead of deleting it.