Standardize integrator variable in ContextCache.

[andrrizzi]

• Global variables can be standardized in ContextCache even without setter/getter.
• Per-DOF variables are ignored for the purpose of computing the integrator hash.
• Add standardization of LangevinIntegrator global variables.
• More global variables and attributes can be standardized by updating the class variable COMPATIBLE_INTEGRATOR_ATTRIBUTES

openmmtools.cache.ContextCache.COMPATIBLE_INTEGRATOR_ATTRIBUTES.update({
    'my_integrator_global_variable': standard_value
})

[pgrinaway]

+1 awesome thanks! Giving this a try now.

[pgrinaway]

Looks like it's working!

[andrrizzi]

Nice! Let me know if you bump into other problems.