set units metadata

[perrygeo]

With support for recording per-band units and descriptions in Rasterio (see https://github.com/mapbox/rasterio/pull/854) we should record the units for the outputs.

ex:

with rasterio.open(output, 'w', **profile) as dst:
    dst.set_units('K')

[sgillies]

@perrygeo it's set_units(bidx, units), so instead:

with rasterio.open(output, 'w', **profile) as dst:
    for bidx in dst.indexes:
        dst.set_units(bidx, 'degK')
        dst.set_description(bidx, "TOA brightness temperature")

[perrygeo]

Units on the TOA products would be "Reflectance" but only if we use raw units or provide a scaling factor to covert back.

[perrygeo]

Note, let's also set the scaling factor in the metadata tags

[perrygeo]

Looking at this more closely, riomucho which has control of the destination dataset object so that will require some changes to that lib. Ticket here meanwhile bumping this from the 0.3 milestone.

One temporary workaround: reopen the dataset in r+ mode after it's all written and set the units. Seems like a hack though...