Update ReadGate/SolutionReadout to handle other solution types

[AlessandroPierro]

Objective of issue:

Lava version:

• [x] 0.2.2 (current version)

I'm submitting a ...

• [x] bug report
• [x] feature request

Current behavior:

• The current way of reading out solutions (in ReadGate/SolutionReadout) only works when the solutions are binary, i.e., QUBO problems. It relies on the assumption that `solution' is the spiking history of the neurons solving a problem and picks the spiking history from 3 timesteps ago, when the minimum cost was actually achieved.

Expected behavior:

• ReadGate/SolutionReadout should be updated to handle general solution types, e.g. integer solutions.

[GaboFGuerra]

This is probably needed for the QP solver.