Include station metadata as timeseries attributes

[pmav99]

It would be quite helpful if the timeseries returned from searvey would contain station metadata as attributes. E.g.

>>> df = searvey.fetch_ioc_station("cuxh")
>>> df.attrs
{'provider': 'ioc',
 'provider_id': 'cuxh',
 'sensor': 'flt',
 'country': 'Germany',
 'location': 'Cuxhaven',
 'lon': 8.7175,
 'lat': 53.867778}

Now, the available metadata might differ for each provider, but provider, provider_id, provider_name, lon and lat should be common in all of the providers.