Add radiation pytest for location

[Siliconrob]

As the value returned from the Solcast API can vary dependent on weather conditions try to do the following

• [x] Look for a result set
• [x] Set your position to location that is dark when you make the request i.e. something like if the current UTC time is 14:00 then that is AEDT 1:00 so it will be dark at lat/lng ‎-33.865143, 151.215256 (Sydney). This is a bit complicated as you will need to find dark time positions for every UTC zone or so or perhaps a minimum of 6 positions on the earth that will be in darkness at any UTC time.

[Siliconrob]

Yay added rough working test in commit https://github.com/Solcast/howto-pandas/commit/7b99600f025c44d221a5e56970e1857bf0ce37f1