0.4.0

• Documented the layout algorithm used in tubes plots.
• Added scale_bar option to demesdraw.tubes(), and --scale-bar option
  to the demesdraw tubes CLI, to draw a scale bar that indicates the
  population size.
• Removed dependency on cvxpy by going back to scipy
  for optimisation of the deme positions. The trust-constr
  method with linear constraints seems to work well.