Update robust glm notebook (#3908)

* updated the Hogg notebook

* attempted to clarify the kwargs in sample() docstring describing how to pass kwargs to the steppers

I believe this fixes #3197

I also noted this need for more clarity in my updated notebook in this PR

`pymc3/docs/source/notebooks/GLM-robust-with-outlier-detection.ipynb`

* Remove deprecated stuff (#3906)

* remove file which is not used

* remove deprecated code

* repair tests and notebooks that used deprecated API

* mention #3906

Co-authored-by: Michael Osthege <[email protected]>

* add deprecation warnings for old backends (#3902)

* add deprecation warnings for old backends

* mention backend deprecation #3902

* fix typo

Co-authored-by: Colin <[email protected]>

Co-authored-by: Michael Osthege <[email protected]>
Co-authored-by: Colin <[email protected]>

* minor formatting to notebook and rework of docstring for sample function

notebook: dropped all headings one level lower to comply with TOC logic, and very minor language edits

sampling.py: clarifired language around single vs compoundstep

* updated the Hogg notebook

* attempted to clarify the kwargs in sample() docstring describing how to pass kwargs to the steppers

I believe this fixes #3197

I also noted this need for more clarity in my updated notebook in this PR

`pymc3/docs/source/notebooks/GLM-robust-with-outlier-detection.ipynb`

* minor formatting to notebook and rework of docstring for sample function

notebook: dropped all headings one level lower to comply with TOC logic, and very minor language edits

sampling.py: clarifired language around single vs compoundstep

* updates folowing AlexAndorra review:

upgrade to arviz=0.7
set prior params to slightly simpler (more justifiable) values, and testvals to simplier defaults
explanatory clarifications
formatting, typos,

* removed the note re step_kwargs, since this PR updates the appropriate docstring

* a cell had become markdown, silly. reset it to code and rerun

* minor code reformatting via black_nbconvert, final check and re-run

* rerun notebook purely as a lazy but safe way to trigger new CI