Generalise topology construction

[garth-wells]

Generalise construction of topology, specifically with respect to ghost cells.

• allows the partitioner to return any set of destinations for each cell
• changes the way Topology is computed, without relying on a band of ghost cells to find the interface between processes
• stores the interface between processes as interprocess_facets in Topology
• removes some assumptions about symmetry in MPI comms
• corrects many errors in comments
• removes some dead code from Topology construction

This may not be a fully complete set of changes required. Additional changes for a subsequent PR:

• fully check over the assumptions in creating shared vertices
• remove GhostMode entirely, leave it up to the partitioner (provide user with built-in partitioners for no ghosting, or facet based)
• provide a utility to compute boundary vertices of a topology

[jpdean]

Looks good to me!