Check Dietrich equation for correctness.

The dietrich_equation() method in the CarpEggs class in asiancarpeggs.py may be incorrectly calculating specific gravity. I am not 100% sure if I implemented it correctly. The get_density() method returns what seems to already be the specific gravity, so that will affect its calculation in dietrich_equation()?

It would also be beneficial to annotate/comment what the variables "Rf" and "temp" are. I was not sure what they were and just copy-pasted from the matlab sandbox adding "temp" to make it a bit more readable.