Add Support for Multi-Member ReCom

[jenni-niels]

This PR introduces a new parent Partition class MultiMemberPartition which adds magnitudes for each of the districts corresponding to their number of representatives. (by default this is set to 1).

Additional changes for computation with MultiMemberPartititions:

• A multimember flag is added to recom proposal to allow the user to indicate their use of such a partition, and that population balancing should be wrt the district magnitude (aka population per representative).
• Constraint function within_percent_of_ideal_population_per_representative added.