Sampler.run_sampler periodic save returns a results.post of all 0

[manduhmia]

Describe the bug
In an attempt to use the intermediate save feature (periodic_save_freq=) in Sampler.run_sampler() the resulting posterior array (results.Results.post) was an array of all 0s.

To Reproduce
Steps to reproduce the behavior:

1. run a normal orbit fit (with >10 steps) and set Sampler.run_sampler(periodic_save_freq=10) to save every 10th step
2. load in the results file when run is complete
3. examine results.Results.post

Expected behavior
Expected an accurately updated posterior of size (N orbits X M params)

Desktop (please complete the following information):

• orbitize! Version 3.1.0