fix issue with template inst float mismatch in slice

[cyrush]

resolves #1659

[cyrush]

@nicolemarsaglia this appears to resolve #1659 , which was reported by a user.

It checks the underlying type to a template type mismatched copy.

That said -- if we actually hit a case with mismatched types it seems like both the old an new path would fail.

I think we may be compiling combos some how that aren't used in practice, might be the nature of the viskores cast and call infrastructure.

I think we should look into if there are better ways to do this with viskores, we may be able to remove some of this code.