Support concurrent/futures interface at top level

[shuds13]

Allows user to run any generator and use libEnsemble to dispatch simulations

[jlnav]

I think based on how most concurrent-futures implementations work, we should support submitting generic python functions with any given args/kwargs to libE workers via concurrent futures. Those functions can still use variable resources, the executor, could perhaps be persistent sims, etc.

We'd need to determine how best to slot in "unexpected" data/datatypes into H. This may involve a refresh-History routine like we discussed with Optimas