Handle absence of TIRS Band 10

[perrygeo]

When the TIRS band 10 is all-zero, as is the case with e.g. LC80460282016097LGN00, the equations 7&8 are filled with nans

    /Users/mperry/env/mapbox35/lib/python3.5/site-packages/numpy/lib/nanfunctions.py:1001: RuntimeWarning: All-NaN slice encountered
      warnings.warn("All-NaN slice encountered", RuntimeWarning)

Which leads to the masking of the entire scene, as if there were 100% clouds.

The fmask algorithm is capable of dealing with the absence of a thermal band. We just need to

• [ ] figure out why some seemingly arbitrary scenes have blank thermal bands (is this a known TIRS issue @celoyd?)
• [ ] adjust the implementation to fallback to a non-thermal mode.