use look-up table for calculating emissivity

[FevetS]

Calculates emissivity for a list of NDVI values and applies the look-up table to an image with Image.remap, which should be faster than calculating emissivity for each pixel according to the GEE Profiler.

[coveralls]

Pull Request Test Coverage Report for Build 66

• 0 of 0 changed or added relevant lines in 0 files are covered.
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Change from base Build 65:	0.0%
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💛 - Coveralls

[cgmorton]

This isn't really relevant now that we have switched to using the Collection 2 LST images, but still an interesting idea to look into if we ever go back to computing LST.