Bug: order of demes with same start times

[molpopgen]

There is a problem with importing demes models:

• If two demes have the same start time, they will be sorted lexically by name.
• This is not correct -- we wish to respect the order in which the demes appear in the file. This order should be the "natural" order of the file -- for the same start time, we use the order given by the iterator in Python.

A minimal example is:

import fwdpy11
import demes

yaml = """
description: single deme model
time_units: generations
demes:
 - name: B
   epochs:
    - start_size: 100
 - name: A
   epochs:
    - start_size: 100
"""

demes_model = demes.loads(yaml)

for g in demes_model.demes:
    print(g.name)

# No burn-in
fp11_model = fwdpy11.discrete_demography.from_demes(demes_model, 0)

print(fp11_model.metadata)

The output is:

python model.py
B
A
{'deme_labels': {0: 'A', 1: 'B'}, 'initial_sizes': {1: 100, 0: 100}, 'burnin_time': 0, 'total_simulation_length': 0}