Added two functions to MeteoData: bool param_exists(string) and void reset()
reset() sets all the values of the MeteoData object to IOUtils::nodata (note: not the date), and param_exists(string) returns true if a certain parameter exists and false otherwise in a MeteoData object.

The Meteo1DInterpolator now correctly creates MeteoData objects that have more parameters than the default ones (like ta, hnw, ...)