Make PEP representations consistent

[donaldcampbelljr]

We currently have 3 ways of representing PEPs:

1. Schema representations -> see: https://schema.databio.org/?namespace=pep&schema=2.1.0
2. peppy.Project object -> this is a mutable mapping containing _config and samples attributes.
3. dict via peppy.Project.to_dict() -> this contains all attributes under the original _config key (but not the _config key!) and the samples.

We would like to ensure that number 3 resembles number 2, so that the data structures have both config and samples attributes.

Finally, in this same process we have decided to rename the config key to project.

In summary:

• ensure to_dict()'s representation matches the peppy.Project representation
• change config key to project
• refactor _samples to samples