Version 0.4.0

• Adds the create_abc_param and create_abc_sumstat functions for converting
  the simulation results into the format needed for abc::abc function (#151).
• Improves the documentation significantly and adds more examples and links to
  help pages (#150).
• Changes name of get_population_indiviuals to get_population_individuals
  (#150).
• Adds an option to active_msms() to download msms' jar file (#153).
• Adds support for partial models. Now, arbitrary sets of features, loci,
  parameters and summary statistics can be combined via + and then be
  added to one or more models later (#155).