Allow interactively running subsets or substeps of defined Ensembles

[jlnav]

Inspired by optimas-org/optimas#118

To perhaps run a small selection of sims as a sanity check, then run larger batches, and/or adjust parameters between smaller ensembles.

[jlnav]

There may be some overlap here with predefining batch sizes or exit_criteria and calling .run multiple times.