Dataprocessing features

Quite a lot minor movements/changes and some data processing tools

1. introduced module proc_functions, containing:

• proc_interpolateMissing - a registered function for interpolation
• proc_resample - a resampler for timeseries, mimicing pandas .resample method, including parameters for "nan_control"
• proc_transform - function to apply timeseries transformations on the data

2. trimmed down the harm_functions module:

• removed from_merged/to_merged
• removed downsample (replaced by proc_functions.proc_resample)
• moved/renamed its _interpolate method to ts_operators.interpolateNANs, since it is used now not only in harmonization context

3. extended ts_operators module:

• interpolateNANs is a usual timeseries operator now and the strings '"linear"' and '"polynomial"', associated with wrappers of interpolateNANs can be passed as arguments to resampling/transformation procedures
• extended transformation string syntax by the $ sign to allow parameter passing. passing "linear$3" refers to linear interpolation of up to 3 values and so on. (See details in doku -> doku comes soo)

4. added test_module for processing functions

• its so boring to write tests