Fix compile warnings that come up with the flags
-Wconversion -Wno-sign-conversion
This catches several instances (mostly in the testing framework) where
types are implicitly converted. I expect these changes to fix some of
the warnings we are seeing in MSVC.
I was going to add these flags to the list of extra warning flags, but
unfortunately the Thrust library has several warnings of these types,
and I don't know a good way to turn on the warnings for our code but
turn them off for Thrust.
Before we assumed that we would only use the basic types specified by
the widths of vtkm::Scalar and vtkm::Id. We want to expand this to make
sure the code works on whatever data precision we need.
