Add asserts to hydromet_stratified.py

[ShaneMPutnam]

It is good practice to add asserts/warningsto the code to make it more robust. For example, a warning could be added if the users enters a PMP that is unreasonably high or enters a return interval that is outside the bounds of the NOAA Atlas 14 precipitation frequency curve. @Mark-S-Bartlett recommends adding asserts/warnings for CN, PMP, and the mu parameter.