Parameter spec API

[maedoc]

Describe the new feature or enhancement

To avoid ad-hoc arrays for configuration of parameter sweeps or priors over parameters (for inference), a new datatype is required which can express the distribution of one or more parameters. This would include "spatial" parameters currently expressed as arrays with one value per region or vertex. Attributes of models or other simulator elements can then accept such Parameter instances instead of raw NumPy array. The downstream simulation code then can make appropriate use of these instances.

A second use for a parameter API is specifying distributions over parameters for the purposes of inference.

Describe your proposed implementation

If the API should be usable by the framework then it may need to be implemented with traits. Otherwise a dataclass implementation may be simpler to do.

Additional comments

An initial partial discussion is taking place at #540