Bug/gamma names

This should fix the gamma names issue. I believe I've found every instance of where `gammap`  and `gammapinv` was called and have added tests to make sure the calling function is correct. I think `gammap` should be renamed to something easier to interpret like `lowerIncompleteGamma`, or maybe `lowerGamma`, but I have not made this change in this PR. 

I made some documentation improvements that might help avoid issues in the future. 
Hm. I'm not in love with the long name. Is there another name used by a different library like R?

Honestly, I had been wanting to analyse different libraries and try to standardize on a naming convention for all these functions.