GenericIOReader cleanup & fix CosmoTools warnings

This commit cleans up some of the code in the GenericIO reader
such that it is more modular. All internal
helper methods have been moved to a separate header,
vtkGenericIOUtilities.h, to make the code cleaner.
Further, the properties associated with querying rank
neighbors (which can be used when reading data in RCB)
have been marked as advanced. Last, a number of compiler
warnings have been fixed.

Change-Id: I8e66b266c89e3dc1e2ae9c4ef0ab8e63893429fc
Merge topic 'gio-cleanup-and-warning-fixes' into next

cdefef7 GenericIOReader cleanup & fix CosmoTools warnings