dict of dict structure for MLR strategy

[alinaciuysal]

As we can't pass complex objects as parameter(s) to obj_fcn, we are currently using global workflow and variable objects. Thus, to allow parallel execution of experiments based on MLR strategy, we need to have this structure in a thread-safe manner.

[alinaciuysal]

will not be fixed until we find a way to properly pass additional, non-objective parameters to function of MBO

[alinaciuysal]

implemented here