refactor normalisation class to handle log_transform

[iluise]

the MR contains the following changes:

• refactored the normalisation and moved to a dedicated class.
• moved all options related to normalisations into a dictionary
• fully backward compatible with the old fields

example:

• for total_precip add the following option: {'corr_type' : 'global', 'log_transform' : True, 'eps': 1e-7 }

solving issue: #84

@mlangguth89 to be used for ztul6nxn and o5n1xbxj. please add the dictionary to the .json file as these runs were trained before I implemented this.