Implement the new Domain construct for CF-1.9

[davidhassell]

The new CF domain variable (cf-convention/cf-conventions#301) requires a CF data model domain construct. In the existing data model, the domain is represented by an abstract Domain concept, but the new CF-netCDF domain variable "promotes" the domain to construct status, on a similar footing to the Field class.

Implementation notes:

• Domain variables can exist independently of data variables, and so cfdm needs to be able to read them from netCDF files.
• Climatological time bounds in a Domain do not have access to any cell methods
• The new Domain construct will need much of the functionality that currently only exists for a Field construct, so refactoring is required to maximise code reuse.