Python-datapath

* add `format()` and `format_iterate()` to support formatting strings in standard Python syntax, but with a datapath for fields.
* slightly update return type of `iterate()`

* A `SplitPath` is now a tuple of `_PathPart` objects, this changes the return type of `split()`
* `iterate()` now throws a `ValidationError` when there is a mismatch between the path part type and the collection type where previously it threw an `InvalidIterationError`; this is now consistent with all other functions.

* Fix: support ranges for `join()`

Initial release