vtkStructuredImplicitConnectivity: fix msvc warnings

MSVC complains about implicit narrowing casts. Use size_t where possible
and static_cast otherwise.

Change-Id: I05016f2085524acb7c1c3ecabc5865509e4e10a2
Merge topic 'fix-msvc-warnings' into master

5acb2307 vtkStructuredImplicitConnectivity: fix msvc warnings