Fix for Issue #24

[Max-Bladen]

Derives from closed and depreciated PR (#198).

Mostly the same implementation, such that:

• plotIndiv.mint.pls() given functionality to process background.predict objects. Included adjusting way in which X and Y axes limits are calculated to ensure polygons show
• mean_centering_per_study() adjusted to add the global scaled:center and scaled:scale attributes to output.
• internal_graphicModule() had the if (style == "ggplot2-MINT") updated to include background polygons in plotIndiv() figures.
• documentation and SemVer updated.

Commits and commit messages were redone too.