* Process langley_mfrsr7nch
* Lots of switching on whether broadband field exists or not
* Some refactoring of channel ordering and number of narrowband channels
* Refactor plotting routine to work with  7nch and also plot actual_wavelength

Passes current langley release tests for bb/6nch configuration.  Runs for
7nch dataset (2018-2020), but output not verified yet.