reduce code duplication for plot layers related to point predictions and intervals

[elray1]

There is some duplicated code for calculating data for plot layers representing 50% and 95% intervals and point forecasts here. Let's reduce duplication by extracting the relevant code into two functions we can call -- one that calculates and returns layers representing point forecasts and one that calculates and returns a layer representing interval forecasts.

[elray1]

I'm reopening this issue, as there's still a substantial amount of code duplication that would ideally be eliminated in this block of code.