Update to use mp.opt_model and mp.set_manager

[rdzman]

MP-Opt-Model now includes classes mp.opt_model and mp.set_manager to replace the functionality of the legacy opt_model and mp_idx_manager classes.

MOST needs to be updated to convert to the use of these new classes in place of the legacy ones that have been deprecated.

See also MATPOWER/mp-opt-model#6.