Resolve distinction between parameters and ECIs

Description

At the moment (v1.2), the icet parameters include the multiplicities. In other words, the parameters are J_i m_i. This is confusing since it contradicts the definition in the icet paper (as well as the convention in the field). It also is confusing when analyzing ECIs.

Tasks

• check all occurrences of "effective cluster interaction" in the code and revise

  grep -i "effective cl" `find . -name '*.py'`
  grep -i ECIs `find . -name '*.py'`
  grep -i "effective cl" `find doc -name '*.rst'`
  grep -i ECIs `find doc -name '*.rst'`

• rename parameters_as_dataframe to to_dataframe
• in to_dataframe include division by multiplicity for ECIs, add parameters to dataframe (=ECI*multiplicity)
• rename plot_parameters to plot_effective_cluster_interactions
• replace savefig in plot_effective_cluster_interactions with plt.show(); check how tests have to be updated (see wulffpack, ase)

Demo

• all tests pass, no degradation in coverage
• check greps above to make sure nomenclature is consistent and correct