Avoid pickle in multiple places
This removes io functionality in several places using pickle.
- Change STM read/write to use JSON (this bit is tested and apparently works)
- Remove untested read and write methods on Cluster
- Remove pickle stuff from bundlemanipulate. The module is untested so it'll be broken if I made a mistake
@schiotz Can you say something about how reasonable this is for bundlemanipulate and Cluster? For Cluster it would probably not be difficult to port to JSON but I can't spend time on unused/untested stuff.