Add pretty Jupyter repr of Projections

[pelson]

Rationale

Replacement of #951 based on review feedback.
Adds a pretty Jupyter repr for Projection instances:

cc: @dkillick

[QuLogic]

So I don't know why you need _repr_html_ if you're now returning None when you can't make the SVG.

[pelson]

I'm not producing just an SVG, I actually want an SVG+repr(projection).

[QuLogic]

There is a downside there that this probably only helps the notebook. I would guess (but don't know for sure) that other viewers like qtconsole might not support HTML (but do support say, PNG).

But also note that every one implemented gets called since the viewers are independent of the kernel, so implementing all of them could be a bit heavy.

[pelson]

There is a downside there that this probably only helps the notebook. I would guess (but don't know for sure) that other viewers like qtconsole might not support HTML (but do support say, PNG).

That is entirely believable. It had crossed my mind to try inserting the repr(projection) inside the SVG body, but I decided that I didn't have enough hours in the day 🤣

[pelson]

@QuLogic - For the sake of tidying up, I suggest we merge this if you are happy with it. @dopplershift can add further feedback after the fact.