TrajectoryAnalysis.resample with include_events=False can't be saved

[ixjlyons]

import condor


class ODE(condor.ODESystem):
    a = parameter()
    x = state()
    y = state()

    dot[x] = -a * x


class Traj(ODE.TrajectoryAnalysis):
    tf = 10
    initial[x] = 1


sim = Traj(a=0.5)
sim_resamp = sim.resample(1.0, include_events=False)

sim_resamp.to_file("sim_resamp.npz")

Traceback (most recent call last):
  File "/home/krlyons/proj/condor/condor/.scratch/save_noevents.py", line 20, in <module>
    sim_resamp.to_file("sim_resamp.npz")
    ~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
  File "/home/krlyons/proj/condor/condor/src/condor/contrib.py", line 768, in to_file
    return implementation.save(model_instance=self, filename=filename)
           ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/krlyons/proj/condor/condor/src/condor/implementations/sgm_trajectory.py", line 577, in save
    model_instance._res.save(filename)
    ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "/home/krlyons/proj/condor/condor/src/condor/solvers/sweeping_gradient_method.py", line 700, in save
    e_idxs = [e.index for e in self.e]
                               ^^^^^^
TypeError: 'NoneType' object is not iterable

[sixpearls]

Closed by #73