Add option to authenticate via proxies and with verification cerificates

[LucaMingarelli]

It is currently not possible to pass proxies and verify parameters upon connection. This imposes substantial restrictions to data access for users working in institutions under proxies.

It would be an easy change amounting to including default parameters proxies=None and verify=True to a number of functions such as

• All calls to requests
• pyam.iiasa.Connection so that this can be called e.g. as conn = pyam.iiasa.Connection(proxies=PROXIES['https'], verify=where())
• pyam.read_iiasa
• etc

Would you kindly be able to push a new version accounting for this?

[znicholls]

I believe PRs are welcome so feel free to contribute the changes needed and ask for a review

[danielhuppmann]

Thanks @LucaMingarelli - we don't have the capacity to implement and thoroughly test such a feature extension ourselves, but happy to review if you can start a PR.