Include indices from xclim

[Zeitsperre]

A feature I would like to see built into ClimateTools would be a few methods for performing agroclimatic indices. Some ideas:

• Growing Degree Days (standard degrees above base in Celsius)
• Biologically Effective Degree Days (Integrating daily min and max with baselines and upper limits)

We could also consider having preset indicators for Heating/Cooling Degree Days (degrees below 18 and above 18, respectively) or GDDs above 0, 5, 10 as they can have engineering and agroclimate purposes, respectively.

I'll be creating a new branch to work on this.

[Balinus]

Can we close this? From memory, you implemented some of them. I don't know if you aim to add more?

[Balinus]

New, more general, title. See how we can link ClimateTools and xlim (https://github.com/Ouranosinc/xclim) which provides a lot of climate indices.