Automatically initialize Kokkos

Calling vtkm::cont::Initialize() is supposed to be optional. However, Kokkos needs to have Kokkos::initialize() called before using some devices such as HIP. To make sure that Kokkos is properly initialized, the VTK-m allocation for the Kokkos device now checks to see if Kokkos::is_initialized() is true. If it is not, then vtkm::cont::Initialize() is called.

Fixes: fides#26 (closed)