Make Job.statepoint read only.

[joaander]

Feature description

Provide one API to access the job statepoint.

Proposed solution

Make Job.statepoint read only with the same performance benefits of cached_statepoint introduced in #975. cached_statepoint would then no longer be needed.

Job.update_statepoint then becomes the only method to change a job's statepoint.

Additional context

This is a breaking change and would require significant changes to the signac documentation and unit tests in a major release.