Feature: Add streaming smoothing and cumulative/incidence derivations to server

[dshemetov]

Fixes #607
Fixes #608

Prerequisites:

• Unless it is a documentation hotfix it should be merged against the dev branch
• Branch is up-to-date with the branch to be merged with, i.e. dev
• Build is successful
• Code is cleaned up and formatted

Summary

Adds support for streaming 7-day averaging and differencing calculations to the Flask server. Relies on #564 for signal name aliasing.

The following signals are currently supported (based on the recent meta-table):

google-symptoms	ageusia_raw_search	TRUE	ageusia_smoothed_search
google-symptoms	anosmia_raw_search	TRUE	anosmia_smoothed_search
google-symptoms	sum_anosmia_ageusia_raw_search sum_anosmia_ageusia_smoothed_search

jhu-csse	confirmed_incidence_num	TRUE	confirmed_7dav_cumulative_num
jhu-csse	confirmed_incidence_num	TRUE	confirmed_7dav_incidence_num
jhu-csse	confirmed_incidence_num	TRUE	confirmed_cumulative_num
jhu-csse	deaths_incidence_num	TRUE	deaths_7dav_cumulative_num
jhu-csse	deaths_incidence_num	TRUE	deaths_7dav_incidence_num
jhu-csse	deaths_incidence_num	TRUE	deaths_cumulative_num

usa-facts	confirmed_incidence_num	TRUE	confirmed_7dav_cumulative_num
usa-facts	confirmed_incidence_num	FALSE	confirmed_7dav_incidence_num
usa-facts	confirmed_incidence_num	FALSE	confirmed_cumulative_num
usa-facts	deaths_incidence_num	TRUE	deaths_7dav_cumulative_num
usa-facts	deaths_incidence_num	FALSE	deaths_7dav_incidence_num
usa-facts	deaths_incidence_num	FALSE	deaths_cumulative_num

There may be others, but these were immediately apparent.

TODO:

• figure out how to use the name aliasing feature and handle "source:*" requests
• choose the row ordering as they are streamed from the database
• make sure "issue" and "as_of" params are handled
• reimplement derivable signal logic: add a "derivable" column to the meta-table, use signal base for the "derivable" signal, and use "is_smoothed", "is_cumulative" to determine which transformations to apply
• change the base signals in the db / meta table from incidence to cumulative

[sgratzl]

@dshemetov apparently if the base branch is deleted and it is a fork GitHub will close the PR instead of changing the base branch. Please open another one

[krivard]

you have write permissions here, please use a branch instead of a fork