Update the nearby wet-dry classification and adding rainfall retrieva…

…l methods from RAINLINK (#129)

* updating tests for nearby wet-dry classification
* allowing "vertical", "Vertical", "horizontal" and "Horizontal" to derive a and b values from ITU table"
* update wet dry calculations
* cosmetic changes in nearby wet dry test script
* adding test funcs for nearby rainfall retrieval
* adding functions for nearby rainfall retrival
* changing the input of nearby wetdry functions from xarray.Datasets to one xarray.DataArray per variable
* updating a and b calculations to take no or one/several channel_ids
* add new example notebook for the nearby link approach