introduction of notebook pyjviz support

[asmirnov69]

This PR is to introduce working notebook support of pyjviz diagrams (examples/notebooks dir). Script mode also works.
The PR is big - a lot of new code files were added to introduce twin diagram objects hierarchy used to create RDF log of method chain execution.
Visualization part of code is based on SVG generated by graphviz. The solution was found how to provide important feature: dataframe nodes can have either .head()-like table or plot visible in node click popup window (this works for both nb and script output version).

I verified that this sequence of install commads in fresh conda env produce right set of packages:

conda install -c conda-forge pyjanitor
conda install -c conda-forge ipython
git clone -b rdfio-intro https://github.com/pyjanitor-devs/pyjviz.git
cd pyjviz
pip install .
conda install -c conda-forge jupyterlab
conda update --all  # somehow fixes the problems when starting jupyter lab.

Examples worth looking at are:

pyjviz/example/notebooks/why-janitor.py
pyjviz/example/scripts

Scripts will create directory ~/.pyjviz and dump output there. one can open .svg result files via browser.
Notebook works without any additional details, don't forget to try to click on DataFrame rectangles.

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