PyTables Light

Implements smoother datetime handling for into and provides a lightweight
wrapper around a `tables.Table` object.

closes #530

- [ ] handle `Time32Col`
In general I find the use of `map, fitler, {key|val}{map|filter}` to often be less well appreciated by general programmers.  They have good uses, particularly when functions are small. When there is a lambda involved though I almost always find the comprehension syntax leads to less grief.