Memory Errors creating large populations of simulated transients

[BlueberryDolphin]

When using MOSFiT to create populations of many transients (~1000 events across 9 bands for 30 days/detections), walkers.json appears to be getting so large that it crashes out with a MemoryError. A possible fix could be to have MOSFiT write each realization to its own json file, and write them out one at a time so that the maximum size of a population isn't dictated by the amount of memory available.