Update all C++ BMI modules to reference virtual destructor

[PhilMiller]

https://github.com/csdms/bmi-cxx/pull/12 has been merged. Now we need coordinated updates of our internal and OWP-wide C++ BMI modules to make sure that they all consistently start to reference the revised definition in clean, atomic steps.

Under the model engine team's purview, there's

• [x] ngen/include/bmi.hpp #832
• [x] ngen/extern/bmi-cxx/bmi.hxx (submodule from csdms/bmi-cxx)
• [x] sloth/extern/bmi-cxx/bmi.hxx (submodule from csdms/bmi-cxx) NOAA-OWP/SLoTH#7

Under (I think) the Formulation and/or Soil team's purview, there's references in

• CFE https://github.com/NOAA-OWP/cfe/pull/122
• SMP https://github.com/NOAA-OWP/SoilMoistureProfiles/pull/17
• SFT https://github.com/NOAA-OWP/SoilFreezeThaw/pull/25
• LGAR-C https://github.com/NOAA-OWP/LGAR-C/pull/26
• PET https://github.com/NOAA-OWP/evapotranspiration/pull/38 - not technically the virtual destructor, but gets in the way when I try to make these updates, so it needs to come along for the ride.
• Topmodel https://github.com/NOAA-OWP/topmodel/pull/51 also an ancillary change

[PhilMiller]

@nels - I'm calling it a night. Let's talk tomorrow about how to coordinate testing and merging the batch that all need to go together.