Define and parse realization config structure for coastal models

[PhilMiller]

Following SCHISM integration study in #547, we need to be able to arrange for a coastal model to be part of a run's realization.

It needs to specify things like

• A name (e.g. SCHISM Lake Champlain or DFLOW Pacific Coast)
• What coastal model to run
• What config file to pass it

An open question is how we'll specify the degree of parallelization of the model. Ideally, realization config would remain independent of process count.