Add Equilibration and PreequilibratedSimulation phases

[lilyminium]

Description

Addresses #593

This PR adds EquilibrationLayer and PreequilibratedSimulationLayers to Evaluator as one solution to #593. The envisioned use is:

• running properties through an EquilibrationLayer to equilibrate within a particular potential energy tolerance
• Use the equilibrated boxes as input to a PreequilibratedSimulationLayer, which runs through all steps of a regular SimulationLayer except box-packing.

This PR branches off #589.

Not currently addressed (perhaps in another PR?):

• relative_tolerance is unsupported for EquilibrationLayer due to its application as relative_tolerance * measured_property.uncertainty, which requires the units to match the original property. Instead, the absolute_tolerance can be specified in kJ/mol for potential energy.
• relative_tolerance may not result in successful properties (issue Relative uncertainty does not result in successful properties unless relative_uncertainty > 1.0 #592)
• properties that aren't Densities or EstimatibleExcessProperties (and only EnthalpyOfMixing has actually been tested) are still unsupported, although it should be easy to extend
• the equilibrated boxes must be present in the storage backend at the moment instead of being in a separate directory. They're not very human-readable either.
• PreequilibratedSimulationLayer requires pre-equilibrated boxes to have been computed with a EquilibrationLayer beforehand (the storage query should check for this in a hardcoded way), to avoid contamination from other stored data.
• have not yet tested re-use of equilibrated boxes across range of force fields
• have not yet figured out what should happen if multiple boxes are present per system in the stored data. Right now there's a hard-coded check for only finding one box. This should get replaced with something that sorts results somehow and picks one.
• It would be ideal to have the EquilibrationLayer also query the storage backend and exit early if a result already exists.

TODO:

• a full test run with properties of concern, and benchmark GPU performance on k8s.
• update the tutorial notebook once it has finished executing slowly on my laptop

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[lilyminium]

Closing for now as this is still very early stages.

[mattwthompson]

This was looking good, but I'll let you decide where and when you want this to go