Convert enums to enum structs

There are multiple examples in VTK-m where a class contains an enum to select a mode. For example, the vtkm::cont::Field class as an enum for the association of a field to points, cells, etc. The problem with using enums of this nature is that the compiler will automatically convert them to ints, which can cause some unexpected behavior.

A better approach would be to use the newer struct-based enums. These will ensure we are using the correct type. So in the case of the associations of field, we would have an implementation that looks something like this:

namespace vtkm
{
namespace cont
{

enum struct FieldAssociation
{
  ANY,
  WHOLE_MESH,
  POINTS,
  CELLS
};

class VTKM_CONT_EXPORT Field
{

Then instead of referencing a value like vtkm::cont::Field::ASSOC_ANY, you would use vtkm::cont::FieldAssociation::ANY.