Define spatial domain in base NF class

[stuart-knock]

Related to #150 (Timeseries) and #133 (parameters)
There are multiple components and desired features that require, or at least can be achieved more easily with, access to explicit knowledge of the spatial domain. Also, the spatial domain is a primary component of neural-field theory. So, it would seem that the logical and practical place for spatial information to live is the base NF class, along with the temporal information (deltat).

Currently spatial domain information is held on Population and used by Wave.

NOTE: Any change of this sort needs to be considered very carefully as any change to NF will propagate through almost the entire code-base.