Refactor: extract make_quantiles from convert_to_output

[djinnome]

convert_to_output is getting a bit unwieldy, and in the course of providing ensemble outputs for the covid19forecastinghub, we found a need to extract the section of the code that computes quantiles into its own function make_quantiles.

Rather than include that code in the sa_ensemble_eval branch, we are separating out that code into its own branch and PR.

This branch will:

• Extract make_quantiles from convert_to_output
• Create a unit test for make_quantiles using dummy data where we know the quantiles.
• Serve as the basis for a future PR where we compute the fluxes in a post-processing step as part of convert_to_output