Simulation export support

[vibraphone]

SMTK does not provide much in the way of export support for simulations; export scripts are expected to do too much low-level work. Pedigree IDs are only available on Exodus models, not discrete models (IIRC). The old GridInfo classes are not meaningful anymore but some queries are not available in SMTK yet, mostly because smtk::mesh is still larval.

There should be a way, from Python, to obtain mesh entities for model entities and relate their boundaries to model boundaries.

The ExportSpec and UserData classes in smtk/simulation should be tested and used or removed.