Fix demes parser zeroing out sample sizes

[nspope]

Fixes #75.

• Using a demes yaml from the CLI is now disabled. The -D flag is recognized but errors out. We should remove it in the future, and update the docs accordingly. Let's do this later once the YAML parser is finished.
• The validation suite for demes now exits early with a message about the -D flag. These should be updated or removed at some later point. I'm not doing this now as there is other testing needed for the YAML parser (for parity with CLI) before re-testing the demes parser.

[nspope]

Not sure why the testing has bottomed out here; going to close and open a new PR