Fixes to boxM, improvements in labeling ellipses

Version 1.8.1

This is a large cumulative release of several versions since the last CRAN release.

• label.ellipse() now returns invisibly the coordinates where the label was placed.
• heplot() adds a label.cex argument to control the size of labels on H/E ellipses. The label.pos argument is vectorized, so that the changes to label.ellipse() now apply.
• boxM() & plot.boxM() now correctly handle the situation where one or more groups have singular covariance matrices because !n>p
• Fixed bug in covEllipses.boxM() method. boxM() now returns ngroups and means among it's components.
• add @family tags to improve cross-referencing in documentation

Version 1.8.0

• describe more fully constructing the datasets table using concept tags
• added text_usr() to add text in normalized device coordinates
• begin to convert documentation to use markdown via {roxygen2md}
• label.ellipse() re-written to allow a fully general way to specify label positions [Thx: Claude]