Allow request headers and other arguments to pass through TDSCatalog

[kahemker]

Is your feature request related to a problem? Please describe.
If the user is working behind a corporate proxy that does not allow for secure SSL traffic to pass, forecast.get_processed_data does not work. You will see errors relating to SSL certificate verification failing.

Describe the solution you'd like
I created a fork for pvlib-python and siphon that show how to pass the necessary arguments all the way through into the session_manager within the TDSCatalog.

Describe alternatives you've considered
I don't see any alternatives. There does not seem to be the necessary global scope variables within urllib to set the necessary arguments. Unfortunately, until my pull request from Siphon is included into that release, my fork of pvlib-python will not run. I tested my new code by pulling in my forked branch from siphon and the test_forecast.py passes.