Set default parameters in processes, not in ProcModels

[phstratmann]

Objective of issue: I found at least one case where default values of Vars are defined in a ProcModel rather than the Process. This should be changed. My example is in generic/models.DiscreteVariablesModel: noise_amplitude = proc.proc_params.get("noise_amplitude", 1) steps_to_fire = proc.proc_params.get("steps_to_fire", 10) importances = proc.proc_params.get("importances", 10)

Rather, set them in the Process and ensure that these hyperparameters will always be defined.

Lava version:

• [x] 0.3.0 (feature release)

I'm submitting a ...

• [x] bug report