PET calculation fixed 365-day calendar

[chrisbanze]

Calculating PET after thornthwaite works great except for one important detail. Having a dataset that uses a fixed 365-day calendar, there is currently no option to do the correct calculation defining all years as no-leap years.

Adding an argument for PET calculation to set the calendar to "gregorian" or in my case "365-days" would be ideal.

I have changed the code in eto.py to not check for leap years and that works, but it would be greatly appreciated if implemented into the package, so that it can be easily shared.