The basic type operators in `Types.h` (i.e. `vtkm::Add`,
`vtkm::Subtract`, `vtkm::Multiply` and `vtkm::Divide`) required the same
type for both arguments. This caused problems when used with `Reduce`
and the initial value type did not match exactly.

Use some tricks from `BinaryOperators.h` to be flexible about using
different types.