Window size should be more flexible

A fixed window size for a corpus does not always make sense, because of a skew in data.
There should be a more flexible configuration open, e.g. providing a function or an array of values.

Start by locating the corpus window-size configuration and the Word2Vec training entry point. Trace how the current fixed value is consumed, then define and verify support for the function or array forms described in the issue, including behavior for skewed data.

Written by the indexing model from the issue text.