Modification to perf_testing.py

Added the following features:

 * The user can now specify "true_mps" instead of being forced to sample it from "true_prior". The current version requires that the user specify ONLY ONE vector of model parameters -- in particular, we must have true_prior.sample().shape == true_mps.shape.

* The performance record now saves the outcomes and experiment parameters. This can come in handy for evaluating the performance of a given heuristic.