Set up initial structure of repo with example data and first methods

[cchwala]

There is already some existing merging code based on poligrain and there is old merging code for radar and CMLs that I have. As a first step in this repo, we should:

• assess current implementations
• discuss best structure for modules (file names, classes, function names and arguments) #15
• add small example dataset
• add basic methods for radar-gauge-CML merging (additive, multiplicative)

[cchwala]

TODO based on discussion about RADOLAN and Kriging-based implementations just now:

• @eoydvin do small PR with example dataset and first implementation (maybe just copy-paste of existing code) for radar-CML merging with some Kriging approach
• @cchwala uses the dataset from the PR above and adds a first version of his RADOLAN implementation
• experiment with both methods
• discuss potential joint structure of functions and/or classes (deadline mid of August)

[cchwala]

Can be closed. Further discussion about structure continues in #15