NEST synapses with distance-based delays

[drodarie]

Context

As a user, I would like to compute dynamically the delays of transmission of spike events through axons based on pre and postsynaptic soma locations.
Currently the NestSynapseSettings delay and weight parameters are separated because these are needed in most of NEST synapse models. The node has also a constants configuration attribute that allows the user to set any synapse parameter with a nest_parameter type.
Currently, there is no way to compute parameters while loading the circuit.

Proposal

• Similar to postprocessing hooks, we should refactor bsb-core.simulation.parameter.Parameter into two abstract dynamic nodes CellParameter, ConnectionParameter that implement a compute function based on respectively the cell_model node or the connection_model node. These parameter classes can be extended to implement any strategy to dynamically load parameters.
• The NestCell and NestSynapseSettings should implement a new dictionary attribute: parameters, which will allow the user to define respectively CellParameter or ConnectionParameter for each model parameter that has to be computed during the simulation preparation. The constants attribute present in both class may remain to maintain the old interface.
• The NestCell and NestSynapseSettings should also implement a boot function that will merge parameters and constants into a single list and ensure that necessary elements are present (e.g. delay and weight for synapses) and no element is duplicated.
• A DistanceDelayParameter class should be implemented, that compute the delays between the pre- and post-synaptic cells of a connection_model based on the axon speed of transmission.

[Helveg]

Great! A small note on

and ensure that necessary elements are present (e.g. delay and weight for synapses)

It's probably best to do this with an easy to override method like get_required_parameters:

required_parameters = ["weight"]
if ...:
   required_parameters.append("delay")

and then call that in __boot__, so that it's easy for subclasses to override that logic