eulerr 4.1.0

Minor changes

• plot.euler() now returns a gTree object. All of the plotting mechanisms
  are now also found in this function and plot.eulergram() and
  print.eulergram() basically just call grid::grid.draw() on the result
  of plot.euler(). This change means that functions such as
  gridExtra::grid.arrange() now work as one would intuit on the objects
  produced by plot.euler().
• Fitting and plotting euler diagrams with empty sets is now allowed (#23).
  Empty sets in the input will be returned as NA in the resulting
  data.frame of ellipses.
• The last-ditch optimizer has been switched back to GenSA::GenSA() from
  RcppDE::DEoptim().

Bug fixes

• The grid parameters available for edges are now correctly specified in
  the manual for plot.euler().
• euler.data.frame() now works as expected for tibbles (from the tibble
  package) when argument by is used.