BUG: mixed sampling causing errors on data loading
See error that arrised in #196 (closed)
mlair/helpers/join.py all keys of
stat_var dictionary is updated if handling hourly data. But this effects not only the dictionary inside the method but also its origin (remember the pointer behaviour of python dictionaries). If MLAir later tries to load daily data for the targets, it gets an error from JOIN's REST-interface, because the request
values is not valid. Replace this single line by creating a new dictionary instead of updating the given one.