Convtools

**Bugfix**

- fixed piping something complex to ``c.join``

**Misc**

- [internals] reworked aggregate & group_by templating
- [internals] reworked optional items processing

**Features**

- introduced ``c.zip``, which supports both args to yield tuples and kwargs to yield dicts
- introduced ``c.repeat`` -- the one from ``itertools``
- introduced ``c.flatten`` -- shortcut for ``itertools.chain.from_iterable``

**Bugfix**

- fixed incorrect aggregate (not group_by) results in case of ``where``
conditions in reducers [convtools-ita 32 ](https://github.com/itechart/convtools/issues/32)

[convtools-ita 30 ](https://github.com/itechart/convtools/issues/30)

**Bugfix**

- fixed nested aggregations

**Misc**

- [internals] reworked aggregate & group_by templating

**Bugfix**

- fixed join + input_arg case

**Bugfix**

[convtools-ita 29 ](https://github.com/itechart/convtools/issues/29)

- fixed right join (conditions were not swapped correctly)