Previously, the items used to identify parts of topology like points,
cells, faces, etc. were in an enumeration. However, they are only really
used in template specialization, and it is easier to use tags in this
case. So, change the enumeration to a set of tag structures. Also made
the following changes:

* Renamed TopologyType to TopologyElement, which is more indicative of
what we are referring to.
* Moved the structures from the vtkm::cont namespace to the vtkm
namespace. There is no reason not to be able to use them from either the
control or execution environments.
* Added a VTKM_IS_TOPOLOGY_ELEMENT_TAG macro to do type checks on
template arguments that are supposed to be topology element tags.