eulerr 3.1.0

Major changes

• The last-ditch optimizer switched from GenSA::GenSA() to
  RcppDE::DEoptim().
• The optimizer used in all the remaining cases, including all circular
  diagrams and initial layouts, was switched back to stats::nlm() again.
• In final optimization, we now use stress instead of residual sums
  of squares as a target for our optimizer.

Minor changes

• label is now a proper generic with an appropriate method (label.euler()).
• The eulerr under the hood vignette has received a substantial update.

Bug fixes

• Fixed warnings resulting from the deprecated counts argument in one
  of the vignettes.
• Fixed memcheck errors in the final optimizer.
• Corrected erroneous labeling when auto.key = TRUE and labels were
  not in alphabetic order. (#15)