Add async method to epidata

[chinandrew]

To be used by covidcast client (to speed up indicator combo) and also in nowcasting.

One thing to note is that running this on some machines and/or in jupyter can lead to this an issue that requires you run these two lines to solve. For that reason I'm not sure we'll want to make this the default method in covidcast, since we don't have much control over people's environments. Adding those lines at the top of the client file seems to resolves it for me, but not sure if that'll hold universally. If it does, then we might be fine. If not, we could set a protected flag in covidcast for async that only our internal users/superusers will change.

Summary of changes:

• Added a method which takes in a list of raw params and sends an async request for each param. Does not do any formatting/cleaning/parsing of the params so it's compatible with any epidata source (at the expense of convenient) 2 helper methods are also added to make the requests.
• Tests

[krivard]

Looks legit, but the documentation for delphi_epidata is so nonexistant that we definitely need people to be able to tell what get/fetch are for just by their names. No accidents from people doing dir(Epidata) and guessing

(do get/fetch even need to be static members or can they be local functions within async_call? will you be calling them directly from nowcast?)

[chinandrew]

Looks legit, but the documentation for delphi_epidata is so nonexistant that we definitely need people to be able to tell what get/fetch are for just by their names. No accidents from people doing dir(Epidata) and guessing

(do get/fetch even need to be static members or can they be local functions within async_call? will you be calling them directly from nowcast?)

Good point, only the async_call is the only one that will be used publicly (maybe i should switch the names of async_call and async_fetch_epidata), so the others don't need to be visible. Ill work on it

[chinandrew]

Updated to make functions local and also renamed them.

[krivard]

👩‍🍳 💋