Rename field associations

The symbols in vtkm::cont::Field::Association have been changed from ANY, WHOLE_MESH, POINTS, and CELL_SET to Any, WholeMesh, Points, and Cells, respectively. The reason for this change is twofold:

• The general standard that VTK-m follows for enum struct enumerators is to use camel case (with the first character capitalized), not all upper case.
• The use of CELL_SET for fields associated with cells is obsolete. A DataSet used to support having more than one CellSet, and so a field association on cells was actually bound to a particular CellSet. However, that is no longer the case. A DataSet has exactly one CellSet, so a cell field no longer has to point to a CellSet. Thus the enumeration symbol for Cells should match the one for Points.

For backward compatibility, the old enumerations still exist. They are aliases for the new names, and they are marked as deprecated, so using them will result in a compiler warning (on some systems).

Fixes #667 (closed)