Convtools

**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)

**Features**

[convtools-ita 28 ](https://github.com/itechart/convtools/issues/28)

- now ``c.iter`` supports ``where`` parameters just like ``c.generator_comp``:

* ``c.iter(c.this() + 1, where=c.this() > 0)``

- now it's possible to use ``.pipe`` wherever you want as long as it lets you
do so, even piping in and out of reducers (``c.ReduceFuncs``)

* e.g. it will raise an Exception if you try to add labels to a reducer input

- now it's possible to use ``aggregate`` inside ``aggregate`` as long as you
don't nest reducers