Change Loihi Learning Rule during runtime in simulation

[weidel-p]

User story

As a user being interested in plasticity, I want to be able to exchange parameters of the learning rule during runtime to avoid the necessity to re-create the complete network.

Conditions of satisfaction

• Parameters of the learning rule can be changed during runtime for the CPU backend
• Parameters of the learning rule create Vars on the Connection and Neuron Process(Models)
• The Vars of the Dense/Neuron Process(Models) have get/set functions
• Tests have been written to demonstrate the expected behavior for each parameter of the learning rules
• Tests have been written for parameters inflicting the post-synaptic neuron
• Code has been reviewed and merged

Acceptance tests

• The get/set functions of all parameters are tested at the Dense Process
• The get/set functions of all parameters are tested at the Neuron Process
• Test for invalid values

Tasks

• Implementation of tests (PW, 1/2 day)
• Implementation of String LavaType (PW)
• Unit tests for String LavaType (PW, 1 hour)
• Implementation of Connection Vars (PW)
• Implementation of Neuron Vars (PW, 1/2 day)
• Documentation (PW, 1/2 day)
• PR and review (PW, GK, 1/2 day)
• Merge to main (PW, 1 hour)