Mcaliley review: better description of fixed versus variable percentiles

percentiles.py has functions for calculating "fixed" percentiles and "variable" percentiles. The difference is explained in site_conditions_examples.ipynb and at Calculations Quick-Reference, but not in the docstrings themselves. It could be helpful to link to Calculations Quick-Reference from the docstrings. Thanks for bringing this to our attention. We have included a link to the Calculations Quick Reference in the docstrings, so they are easily accessed by users.