Fixes #132.

* vcsn/labelset/letterset.hh,
* vcsn/labelset/nullableset.hh,
* vcsn/labelset/tupleset.hh,
* vcsn/labelset/wordset.hh:
Add rdivide to labelsets.

* vcsn/algos/divide.hh: Add dyn bridges.
* vcsn/dyn/algos.hh: Add dyn declarations.
* python/vcsn_cxx.cc: Add python bindings.
* tests/python/divide.py: Test them.
* python/vcsn/label.py: Add / and // operators.

* doc/notebooks/Algorithms.ipynb,
* doc/notebooks/label.ldivide.ipynb,
* doc/notebooks/label.rdivide.ipynb,
* doc/notebooks/label.operators.ipynb:
Document.

* doc/notebooks/local.mk: Update makefile.

* NEWS.md: Mention this in news.