Rename FN_MANIFEST

[bdice]

Suggesting this change for signac 2.0:

• Job.FN_MANIFEST and similar properties of Job/Project should be private.
• "Manifest" should be changed to "state point" universally in variable names and docstrings.
• e.g. FN_MANIFEST should be _FN_STATE_POINT.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[vyasr]

Still relevant, will be addressed as part of #527.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[vyasr]

This issue was addressed by #610. Unless there's a reason to revisit the visibility of the attributes, I think we can close this. @bdice what do you think?

[bdice]

I'd still like to make these attributes private. I can handle creating a PR.

[bdice]

I thought about this some more. Maybe these should remain public? I don't have strong feelings so maybe this is better to close and leave as-is.