Integrate SOLPOS

[dcposch]

Now that SOLPOS is ported and has a unit test, use it replace the following:

• Simple solar position calculation. SOLPOS is much more thorough, better-tested, and takes refraction and other effects into account.
• Insolation constant. In reality, insolation is not a constant, atmospheric attenuation makes it vary over the course of the day. SOLPOS apparently has good models for this.