Restoring LangevinIntegrator from a compound integrator fails to restore hidden variables

[nathanmlim]

Example below

from simtk.openmm import CompoundIntegrator
from openmmtools.utils import RestorableOpenMMObject
from openmmtools.integrators import *
temperature = 300*unit.kelvin
collision_rate = 1/unit.picoseconds
timestep = 1.0*unit.femtoseconds
n_steps = 100
alchemical_functions = {'lambda_sterics' : 'lambda'}
ncmc_integrator = AlchemicalNonequilibriumLangevinIntegrator(alchemical_functions, 
                                                             splitting='O { V R H R V } O',
                                                             temperature=temperature,
                                                             collision_rate=collision_rate,
                                                             timestep=timestep,
                                                             nsteps_neq=n_steps,
                                                            measure_heat=True)
integrator = LangevinIntegrator(temperature=temperature, 
                                        timestep=timestep,
                                       collision_rate=collision_rate,
                                       measure_heat=True)
print(integrator)
print(integrator._measure_heat)

compound_integrator = openmm.CompoundIntegrator()
compound_integrator.addIntegrator(ncmc_integrator)
compound_integrator.addIntegrator(integrator)
print(compound_integrator)

langevin_integrator = compound_integrator.getIntegrator(1)
RestorableOpenMMObject.restore_interface(langevin_integrator)

print(langevin_integrator)
langevin_integrator.reset()
#print(langevin_integrator._measure_heat)

Specifically, this makes it so I am unable to reset the integrator. (Error below):

>>> <openmmtools.integrators.LangevinIntegrator; proxy of <Swig Object of type 'OpenMM::CustomIntegrator *' at 0x15182b6480> >
>>> True
>>> <simtk.openmm.openmm.CompoundIntegrator; proxy of <Swig Object of type 'OpenMM::CompoundIntegrator *' at 0x15182e4060> >
>>> <openmmtools.integrators.LangevinIntegrator; proxy of <Swig Object of type 'OpenMM::CustomIntegrator *' at 0x15182e4600> >
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-6-9c345ca3462e> in <module>()
     30 
     31 print(langevin_integrator)
---> 32 langevin_integrator.reset()
     33 #print(langevin_integrator._measure_heat)

~/Research/Github/molssi-project/openmmtools/openmmtools/integrators.py in reset(self)
   1186         """Reset all statistics (heat, shadow work, acceptance rates, step).
   1187         """
-> 1188         self.reset_heat()
   1189         self.reset_shadow_work()
   1190         self.reset_ghmc_statistics()

~/Research/Github/molssi-project/openmmtools/openmmtools/integrators.py in reset_heat(self)
   1163     def reset_heat(self):
   1164         """Reset heat."""
-> 1165         if self._measure_heat:
   1166             self.setGlobalVariableByName('heat', 0.0)
   1167 

//anaconda/envs/molssi/lib/python3.5/site-packages/simtk/openmm/openmm.py in <lambda>(self, name)
  10724     for _s in [Integrator]:
  10725         __swig_getmethods__.update(getattr(_s, '__swig_getmethods__', {}))
> 10726     __getattr__ = lambda self, name: _swig_getattr(self, CustomIntegrator, name)
  10727     __repr__ = _swig_repr
  10728     ComputeGlobal = _openmm.CustomIntegrator_ComputeGlobal

//anaconda/envs/molssi/lib/python3.5/site-packages/simtk/openmm/openmm.py in _swig_getattr(self, class_type, name)
     72 
     73 def _swig_getattr(self, class_type, name):
---> 74     return _swig_getattr_nondynamic(self, class_type, name, 0)
     75 
     76 

//anaconda/envs/molssi/lib/python3.5/site-packages/simtk/openmm/openmm.py in _swig_getattr_nondynamic(self, class_type, name, static)
     67         return method(self)
     68     if (not static):
---> 69         return object.__getattr__(self, name)
     70     else:
     71         raise AttributeError(name)

AttributeError: type object 'object' has no attribute '__getattr__'