Add parameter value per cluster to CE dataframe

Description

Currently the parameters generated by the optimization and stored in ClusterExpansion objects are not the J_{\alpha} in the famous CE formula but actually J_{\alpha} m_{\alpha}. For plotting purposes and analysis it would be useful to get the J_{\alpha}.

Subtasks

1. Add this as a column to the CE dataframe,
2. also make this a property of the ce.
3. default to use these parameters in the ce.plot_parameters
4. update documentation and add clarification

Demo

• tests pass
• documentation compiles
• distinction between parameters with and without multiplicities clarified in documentation