Fix usage of undefined ivar MPIComunicator

This commit fixes an issues that resulted after merging two
conflicting branches on next. Specifically, the MPICommunicator
ivar is no longer a member of the vtkPGenericIOReader class.
Instead, is an ivar of the internall metadata data-structure
that is defined in the implementation.

Change-Id: I5522b79683d99fe8fb0f31f5479727347bc20ac5
Merge topic 'fix-cosmotools-python-wrapping' into next

a2b383e Fix usage of undefined ivar MPIComunicator