Calculate pressure and temperature from elevation

I added a few functions for calculating pressure (in pascals) and temperature (in K) from elevation.

Also added some unit tests to testsolar.py for the new calculations.

The pressure calculation is particularly useful for use with functions allowing a millibar value, letting you get a better estimate of millibars for a given elevation.

The calculation of temp based on elevation is thrown in because I use it in my tree simulation stuff. Since I'm starting to use pysolar as a part of my simulations, it seemed like a good place to live. :)