consolidating various tools for language sequencing

Originally this work (some of which dates back over a decade) was going to be in vocabulary-tools but given you can sequence items other than vocabulary (like morphology or syntactic constructions), it seems better to have a separate toolset.

I'm focusing initially on the algorithms and Python API using GNT data (temporarily included in the repo). But the goal will be to support any text (without or without linguistic annotation) and to provide a command-line tool and (eventually) web interface.

Work on this code is largely motivated by the Greek Learner Texts Project.

All code is made available under an MIT license.

For more of my work on Ancient Greek, also see http://jktauber.com/