GSODWeatherSource and TMY3WeatherSource should have options for loading a local file

[philngo]

Normally loading a weather source looks like this:

GSODWeatherSource(station_id,start_year,end_year)

But there should be an option that either looks like this:

GSODWeatherSource(station_id,start_year, end_year,data_dir=[])

or like this:

GSODWeatherSource(filenames=[])

Or maybe there's another interface that would work better

[philngo]

This may be less important since caching seems to be working well for most users.