nzilbb.vowels 0.4.2

• In plot_pc_vs(), allow unicode letter class and mark combinations in
  variable names
• More detail provided for article on use of rotations.
• Remove dependency on vegan::procrustes()
• Fix typo in plot_correlation_counts()
• Switch to institutional email due to unreliability of previous email and
  consequent archiving of 0.4.1.