Create small lib to calculate global warming level

This MR uses the code taken from a jupyter notebook by @m212075 and @g300050 to create a small python library that can be used to calculate global warming levels from cmip5 and cmip6 model data. The lib can use either intake or freva to search for data. There is also command line interface (cli). Usage examples are given in the docs.

I had to adjust the code a little mainly because cmip5 models were showing a bump around the year 2000 -> cmip5 apparently had their very own y2k problem.

I've already built in the code into a plugin and it seems to work fine. Thanks, guys for the source and inspiration.