better air mass ratio model?

[aqreed]

Hello @pingswept ,

I have been working lately on my own solar irradiation model, and I believe this air mass model based on Kasten & Young (1989) or this one, based on Young (1994), might be interesting for this package too.

If it is interesting for you indeed, I can open a pull request, provided it does not break anything at testing.

BR,
A.

[pingswept]

If it doesn't break the validation code we've got, sounds great. I would be hesitant to include other models if it breaks our validation, and we have no better way of validating the new code.