vtkm::Vec<T,N> should support constexpr construction.

As we continue on with C++11+ support it is becoming more common to want to construct vtkm::Vec types at compile time ( mainly for testing ) like the following:

constexpr vtkm::Vec<vtkm::UInt8,3> expected = { 0, 255, 0 }; 

We should refactor vtkm::Vec to make constexpr constructors possible.