Adding user_key to Data Containers

[daliu87]

Adding a new attribute to all Data Containers that is preserved through hdf5 saving, a user_key attribute to track any user specified key, such as the parameters used in parametric grid search.

This requires the modifications of all data container constructors and helper constructors. This won't be done all at once, starting with Posteriors first since it's the most saved container.

This is the downside of a completely inheritance based hierarchy of data types. Should look into composition over inheritance...