BMI Model instantiation protocol

[PhilMiller]

From a meeting about parallel extensions, we found that we'd really like to have an agreement on how objects that implement BMI should be instantiated by driver/client code.

[PhilMiller]

The National Water Model NextGen framework has adopted an approach to this roughly described in https://github.com/NOAA-OWP/ngen/blob/master/doc/BMI_MODELS.md

From the meeting, it sounds like some of the NCAR code has taken a somewhat different approach, that hopefully a member of that team can point to docs for or describe.