Add getRiseAndSetAtSolarAngle method

[MatthiasKunnen]

I have a use case where I need to get the time on which the sun will be at multiple angles. Since these altitudes are unknown I can't really name them. Therefore I've added the getRiseAndSetAtSolarAngle method.

The method can be seen as a shortcut to addTime followed by getTimes that does not pollute the times array. It saves a method call and does not calculate any other times when it doesn't need to.

[MatthiasKunnen]

@mourner, are you accepting PRs at the moment?

[mourner]

Sorry for not reviewing this in time — I got swamped by other commitments and have fallen behind on this project. :( Closing as the PR is superseded by #137, which is more accurate and complete.

[MatthiasKunnen]

@mourner, that issue seems to address a different issue. #137 changes the height of the observer. This PR allows passing of the angle of the sun you want to know the sun rise and sun set time of. Am I incorrect and does #137 provide a way to do this?

[mourner]

Ah apologies, I misread the description — reopening.

[MatthiasKunnen]

The name might be confusing indeed. What do you think about getTimesAtAngle?

[MatthiasKunnen]

@mourner, I've made some additions:

• Rename the method to be more intuiitive
• Add elevation parameter as was added to getTimes
• Accounted for atmospheric refraction
  It's not 100% correct. I tried using the astroRefraction function but that seems to be off by a a few seconds. I'll add a commit to showcase this

[MatthiasKunnen]

Any idea why I need to multiply by 100?

[gtim]

This saved me some time, thank you for sharing your solution @MatthiasKunnen !

[Hypnos3]

published a new version which include this as https://github.com/Hypnos3/suncalc3