Mark checkpointing as experimental and add more tests

[RudolfWeeber]

No description provided.

[RudolfWeeber]

The docs now clearly state the limitations and limited testability.
I'm de-milestoning this, but we should add an end-to-end test with a reasonably complex simulation setup in the long run.

[KaiSzuttor]

@RudolfWeeber have you checked the coverage of the checkpointing tests?

[RudolfWeeber]

On Tue, Aug 06, 2019 at 01:06:44AM -0700, Kai Szuttor wrote: @RudolfWeeber have you checked the coverage of the checkpointing tests?

Up to a point, yes. It includes the main integrator features, standard script interface classes, ScriptObjectRegistry-derived script interface classes such as LBBoundaries and Constraints. Also @pkreissl and me have run end-to-end tests on our current research projects.

[jngrad]

Tests were added for constraints (#3105), thermostats and integrators (#3245) checkpointing in 4.1.1.

[jngrad]

Progress report:

• checkpointing testing has improved considerably since 4.1.1
• multiple combinations of features are tested to detect interactions (e.g. due to the order in which global variables are reloaded)
• accidental double checkpointing of global variables has been sorted out (Remove globals.pyx #4276)
• non-checkpointable features raise an exception when checkpointed (Catch ObjectList checkpointing bug #4287)