BME680 Driver + BSEC

[scalablecory]

The dotnet/iot package already has a BME680 driver, but to be of any use the BSEC library is needed.

This needs:

• A small C library that provides a P/Invokable layer. BSEC is a static library that it must pull in.
  • It looks like our own .so can be distributed but not the static library. So whatever is checked in must put the BSEC .lib in a .gitignore.
• A small P/Invoke interface to the BSEC library.
  • This should include the former made .so, directly in the repo.
• An ObservableSensor implementation for BME680 that uses the BSEC library.

This looks like it can reduce the overall part count and price by replacing the SGP40 and MS5637.