Add Normalized Burn Ratio (NBR) composite example

[djhoese]

Sam Batzli would like to use an NBR composite. Rather than having to compute it from floating point or 16-bit geotiffs, it may be better to compute it as a Satpy composite by configuring it in a YAML configuration file. Here's my attempt.

Definition: nbr = (nir - swir) / (nir + swir)

YAML to put in etc/composites/viirs.yaml:

  nbr:
    compositor: !!python/name:satpy.composites.viirs.GenericCompositor
    prerequisites:
      - compositor: !!python/name:satpy.composites.RatioCompositor
        prerequisites:
          - compositor: !!python/name:satpy.composites.DifferenceCompositor
            prerequisites:
              - name: M11
              - name: M07
          - compositor: !!python/name:satpy.composites.SumCompositor
            prerequisites:
              - name: M11
              - name: M07
    standard_name: normalized_burn_ratio

Produces:

Note: This is dynamically linearly scaled. I'm sure there is something else people would expect.