Update documentation and scripts for preprocessing

[SimonTreu]

In readme.md

• [ ] Add specification which data are to be downloaded from the ISIMIP website to reproduce the results from the paper
• [ ] How (and why) to merge data
• [ ] Preprocessing steps are currently mixed up in the chapters Usage and Preprocessing. Re-organize

In the code

• [ ] It should be documented more clearly in which order preprocessing files must be run
• [ ] Remove all hard-coded paths into a common config file
• [ ] Add wrapper that combines all preprocessing scripts. Optimally this should run indipendend parts (e.g. different variables) in parallel.
• [ ] Write a description of the requirements on attrici input files