Convtools

**Bugfix**

- fixed ``c.item(..., default=c.input_arg("abc"))``-like cases, where input
args passed to item/attr with defaults

**Bugfix**

- fixed ``c.group_by`` case without reducers like:
``c.group_by(c.item(0)).aggregate(c.item(0))``

**Misc**

- removed unnecessary ``debug=True`` enabled by default for ``join`` conversions

**Bugfix**

[convtools-ita 24 ](https://github.com/itechart/convtools/issues/24)

- fixed populating ``linecache`` with source code (previously new lines were not preserved) -- debugging issue

**Features**

[convtools-ita 23 ](https://github.com/itechart/convtools/issues/23)


- improved reducers to be usable on their own

python
c.aggregate(
c.ReduceFuncs.DictSum(
c.item("name"),
c.item("value")
)
)


previously it was possible to use them only within ``c.reduce`` clause:

python
c.aggregate(
c.reduce(
c.ReduceFuncs.DictSum,
(c.item("name"), c.item("value")),
)
)


- allowed piping to reducers, still allowing to pipe the result further

python
c.aggregate(
c.item("value").pipe(
c.ReduceFuncs.Sum(c.this()).pipe(c.this() + 1)
)
).gen_converter(debug=True)


- fixed nested piping in aggregations
- reworked docs to use testable code

**Misc**

- improved pylint rating
- added a few type hints
- added a few docstings