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