Implementation of UGRID (part 1 of 2) #696

CF-1.11 will recognise the UGRID conventions (cf-convention/cf-conventions#153), and so cf-python needs enhancing to implement the extended CF data model that this implies (specifically, the new Domain Topology and Cell Connectivity constructs).

This work relies on cfdm implementing the new data model - see NCAS-CMS/cfdm#270 and NCAS-CMS/cfdm#271

This part (1) of this enhancement deals with the data representation, and collapsing and regridding of meshes.

Part 2 (#697) will implement the ability to write UGRID datasets to disk.