[Feature] SPR time series (AM)

[Schiano-NOAA]

One of the high priority figures for development in {stockplotr} is an spr timeseries figure. We do not have a current example for this so the developer for this table has some free rein to choose the design and quantities.

Start: look at the variety of spr figures available

Next: work on setting up a branch (feat-plot-spr) and a new function called plot_spr() for this table. Please follow the branch and PR indicators. There are instructions in the CONTRIBUTING.md that describe how each table or figure are set up and the steps/related functions that can create it.

Last: set up a testing framework using {testthat} to test the function.

@Schiano-NOAA or @sbreitbart-NOAA can run tests on the function to ensure it is compatible with all models that are compatible in the convert_output function. (We are planning on adding test data sets to the package for all models in the future.)

Please complete what you are able to do and the package maintainers can pick up where you left off.