The CMake flag and define differ in their capitalization of the 'm' in
VTKm so I've made CMake variables that match those used in Configure.h.in,
thereby keeping the original naming of VTKm in CMake code, and VTKM in the
C++ code.

This fix also revealed some areas in CellSet and CellSetExplicit where ints
where used instead of vtkm::Ids which caused errors with child classes who
override the methods and returned a vtkm::Id instead of an int.

Also fixed issues that appeared in TestOutOfMemory which got out of date due
to not being compiled since the `VTKM_USE_64BIT_IDS` flag would never be set.
The test now runs and passes when 64bit ids are enabled.