Fix inconsistent methods syntax (read/write, get/set, etc.)

[aimalz]

Under "storing/retrieving ensembles" in the demo, there's a mismatch between the code and the description (qp.Ensemble.read_from vs. qp.Ensemble.read).

Also, since the primary use case is RAIL, we should make a shortcut to opening a DataHandle directly without having to manually run it through tables_io. Perhaps this could be the difference between read/write and read_from/write_to, whether it's just dealing with the raw qp.Ensemble or the DataHandle?

EDIT: #192 covers a broader issue with qp.Ensemble.ancil but also has an example of .set() without .get(), which effectively has a totally different syntax of just .ancil(). It would be a lot more intuitive to users if we were consistent about using or not using these pairs of common methods, and it might prevent some pathological behavior at the same time.

[eacharles]

Neither qp.Ensemble.read_from nor qp.Ensemble.read exist.

qp.factory.read does exist, and it is equivalent to qp.read. I'll go ahead an update the demo to use that.

[eacharles]

I'm not understanding what you want with data handles and tables_io here.