Add calculated values to `model` objects

[grinnellm]

Start a new R script "calcs.R" and make functions to calculate some values:

• Effective harvest rate (calc_harvest_rate())
• Productivity and productivity rate (calc_productivity())

The input would be the models object as it has everything required for the calculations. Instead of returning a tibble, add these calculated values to the models objects (for major SARs), for example major_models[[1]]$calcs$hr in the same format as major_models[[1]]$mcmccalcs$sbt.quants (i.e., 5%, 50%, 95%, and MPD by year). Then use these calculated values to make figures and tables (and text in get_vars), instead of calculating them each time we need them.

Doing these calculations once would speed things up and be less prone to errors.