Updated original nbd data to have a pandas Dataframe format
Wrapper in core.py for adding expressions to models
Modification of build_model_multiconf to use expressions
In-progres version of likelihood using expressions and old way
Expressions-based implementation of nbd_plate_mcmc
Moved old notes on linear model to docs
Moved nbd_linear_model to /old
Revised setstate for NBDPlateMCMC to use new likelihood

Instead of get_likelihood_function(), as in the parent class.
Moved nbd_parallel_model and nbd_model_shared to /old
Moved old_two_cpt to /old, saved its notes in core.py docstring