Adding an option to output loadings plot in plot_pca()

[gtluu]

Hi,

I've noticed that there was no option to view a loadings plot in the plot_pca() function, so I have been working on making some minor changes to the code to support this feature:

• A new loadings parameter has been added to the plot_pca() function.
• When loadings = TRUE, a ggplot object or dataframe will be the output of plot_pca() depending on whether the plot parameter is true or false.
• geom_point is not used for the loadings plot; instead, label=TRUE regardless of user settings when calling plot_pca()
• Changes have been made to the documentation comments.
• Added a line in to test_8_plot_functions_explore.R to ensure that loadings = TRUE and `plot = TRUE' returns a ggplot object.