Consider thread safety

[StokesMIDE]

Thread safety is in idelib is minimal and inconsistent. It's not (yet?) a serious problem, but it could be in the future. Some handing of race conditions was added to address specific problems, but it's a hack. Use a threading.RLock around adding/ending a Session, updating/using Transforms, appending/accessing EventArray data, etc. Also consider making some flag variables (like Dataset.loading) properties that get/set a threading.Event.

[StokesMIDE]

Note: Some of the race conditions that were hacked around in the past were resolved after changing the EBML library to ebmlite. Existing thread-safety things should be reexamined.

[CFlaniganMide]

An update regarding thread safety:

As of #49 , Each channel has a lock used when reading or writing data. Specifically, the EventArray methods append, accessCache, and fillCache when manipulating the _data member. This should be added in more places, but it's a good start.