Lukas issue032 feat plotting postprocessing

added some docs, new plot routine plot_competitive_skill_score, skill score plots are now included in postprocessing, many changes and bugfixes in skill score calculation

bugfix: added start and end to the default kwargs keys, before the time range in all subsets (train, ...) have been equal!

added 4th subset "train_val" that consists on the union of train and val. Is needed for the external skill score calculation. Also added simple MSE function (no need to install sklearn just for mse)

first part of skill score calculation (CASE I and II implemented), added copy of skill_score_plot from felix repository (not checked yet)

minor change in data loading process, there can be now the option to re-download data during preprocessing

created detailed description of all data paths (which steps creates which data and where is it stored)

post processing can load local model if no new training was performed and trained model was saved locally for the regarding experiment

post processing can plot station_map(), but countries are not filled with e.g. elevation data (just blank fill)