VolMesh public accessor and query methods

# Feature Request

Being able to query VolMesh the same way as Mesh for elements, predicates, etc.
Public query methods being the same for VolMesh as for Mesh makes for an intuitive interface.

- Some accessor/query/... functionality is not yet implemented for VolMesh (_halfface.py_),
although it's naturally a superset of accessor functionality of Mesh (_halfedge.py_).
**-> for DRY-ness, could accessor/query methods be implemented as a mixin or shared base class?**

in _halfface.py_ :
```python
def vertices_where(self):
raise NotImplementedError

def edges_where(self):
raise NotImplementedError

def faces_where(self):
raise NotImplementedError
```

- key-index maps getting the same public aliases in _halfface.py_ as in _halfedge.py_ :
```python
def key_index(self):
return {vertex: index for index, vertex in enumerate(self.vertices())}

vertex_index = key_index

def index_key(self):
return dict(enumerate(self.vertices()))

index_vertex = index_key
```