faster process_cmip_data: allow flattened cmip and selected time series

[lilianschuster]

Allows to use flattened CMIP data in process_cmip_data. The option is at the moment only used for the flattened overshoot and stabilisation options of gfdl-esm4 that go until 2500. This method is probably faster than always processing the unflattened files.

[lilianschuster]

allows to only process a selected time period of the GCMs for process_cmip_data and process_isimip_data.

Due to the rolling window for the std. bias correction of the temperature, the selected time period needs to be larger by +/- window_years_length/2 to create the exact same time series as without y0 and y1. That means, if bias corrections is done for e.g. from 2000-2019, need to set y0=1990

• tests added

[fmaussion]

@lilianschuster thanks! Please document this in the notebooks