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.