Dark sky solar data

[JrtPec]

Dark Sky has added Solar Irradiance data as a beta.

Note:

• The values are calculated, not measured. Dark Sky uses the position of the sun in combination with cloud cover.
• Western Europe is not in Dark Sky's "primary region", therefore the data is not super-accurate.
• Since it is a beta, the algorithms and therefore the values may change
• I have done a qualitative analysis that compared these values with those measured by KNMI (Netherlands). The differences were significant (27% lower). I have notified Dark Sky and they will investigate and possibly update their algorithms.
• You need to delete your cached files in order to include these new values (everything will have to be re-downloaded)
• If Dark Sky were to update their values, the cache needs to be deleted again.

[coveralls]

Coverage decreased (-0.2%) to 69.493% when pulling 8e837db858e39aaa52f4c5e92a0aa4b8b59019d2 on JrtPec:dark-sky-solar-data into aa994e71a9b36f2c726f966e38ffec261136542a on opengridcc:develop.