Implement a functionality so that users can provide the climatological values for GAMMA in the steps blending

[sidekock]

In the folliwing part of the code something should be added/changed: Get the climatological lag-1 and lag-2 auto-correlation values from Table 2 in BPS2004. Hard coded, change to own (climatological) values when present. "#" TODO: add user warning here so users can be aware of this without reading the code?

This todo summarized the current problem. First suggestion by @RubenImhoff is to: Should we build in a functionality so that users can provide the climatological values for GAMMA, e.g. through the kwargs?

Recomended to only start working on this once the refactor-steps-blending is merged into the master