0.1.0a4

Breaking changes:

• The deme id field has been renamed to name. This applies to both
  the data model (YAML files) and the Deme class.
  (#246, discussion at popsim-consortium/demes-spec#59)

Bug fixes:

• Check for multiple pulses causing ancestry proportion > 1.
  (@grahamgower, #250, #251)
• Check selfing_rate + cloning_rate <= 1.
  (@grahamgower, #242, #251)
• Check for pulse time edge cases.
  (@grahamgower, #243, #249)
• Check sum of migration rates entering a deme are <= 1.
  (@grahamgower, #244, #249)
• Fix migration.end_time in convert.from_msprime.
  (@grahamgower, #241)