2023 Release and added functions

This release represents an update to the package to access the most recent release of BBS database (2023 - including field observations from 1966 - 2022).

In addition, there have been some fixes and additional capacity that relate to the following issues:
#10 adds some code to the first_difference, spatial model to force 0 spatial variance in 2020 - the year when the BBS was cancelled and so there are no data and therefore no information with which to meaningfully estimate the variance in annual differences among strata.
#50 allows the plot_map() function to plot any of the numerical values in the output from generate_trends(). e.g., to display trend uncertainty by plotting the credible limits or width of the credible intervals of the trend estimates, or to display the mean relative abundance among strata during the trend period. Example added to the Advanced Vignette.
#52 newest database release to include field observations from 1966 - 2022.
#26 allows for the use of highest posterior density intervals in the generate_indices() and generate_trends() functions. HPDI often provide a better description of the posterior distribution, particularly for skewed distributions, such as those from a log-transformed predicted count (i.e., the indices of annual relative abundance). Example added to the Advanced Vignette.
#41 run_model() function by default now cleans up the .csv files created by Stan, once the model has finished and has been saved to .rds file (also done by default). Saves local disk space, as once the model output is saved in the rds file, the csv files are redundant.