Combine declination formulas into one like airmass

[mikofski]

Ditto for equation of time. See relative airmass and extra radiation for examples

[wholmgren]

What declination formulas are you referring to?

[mikofski]

Spencer '71 and Cooper '69, both in PR #276, ditto for eot

[wholmgren]

#427 is pushing the get_* pattern for wrapper functions. I think it would apply here too.