Demes

**Breaking changes**:

- The interpretation has been changed for symmetric migrations when the
`start_time` (and/or `end_time`) is not specified. Symmetric migrations are
now resolved separately for each pair in the list of participating demes.
To accommodate this semantic change, the `SymmetricMigration` class has
been removed, and symmetric migrations are always resolved into pairs of
`AsymmetricMigration` objects.
({user}`grahamgower`, {issue}`263`, {pr}`268`)

- The `size_function` field can no longer be an arbitrary string.
Only the "constant" and "exponential" strings are recognised.
({issue}`262`, {pr}`278`)

**New features**:
- The `from_ms()` function has been added to convert an ms command line
into a `Graph`.
({user}`jeromekelleher`, {user}`grahamgower`, {issue}`74`, {pr}`102`)

**Bug fixes**:
- `Graph.in_generations()` no longer changes time values for a graph
when `time_units == "generations"` and `generation_time is not None`.
({user}`grahamgower`, {issue}`273`, {pr}`274`)

**Breaking changes**:

- The deme `id` field has been renamed to `name`. This applies to both
the data model (YAML files) and the `Deme` class.
({pr}`246`, discussion at https://github.com/popsim-consortium/demes-spec/issues/59)

**Bug fixes**:

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

**Bug fixes**:

- Fix `Graph.in_generations()` to also convert the `Deme.start_time` field.
Thanks to {user}`apragsdale` for reporting the problem.
({user}`grahamgower`, {issue}`224`, {pr}`225`).
- Fix `assert_close()` and `is_close()` equality checks to compare the deme
`start_time`.
({user}`grahamgower`, {issue}`224`, {pr}`225`).

Alpha release for testing. The API and the schema for YAML files have been
largely agreed upon. Backwards-incompatible changes before the first stable
release are still possible, but are considered unlikely.

Initial alpha release to reserve the name 'demes' on pypi.