Prequel to if/else changes to mpl circuit drawer

[enavarro51]

Summary

Needed changes to base mpl drawer before implementing if/else display.

Details and comments

This PR changes some of the structure of the mpl circuit drawer as a prelude to providing a full display of the interior of if/else gates. About 200 lines of code are changed and most are repetitive variable name changes. The changes are

• Remove the Anchor class from the drawer. This class was mostly redundant and just added complexity. The plot_coords method and the x_index attribute were retained and are now part of the MatplotlibDrawer class.
• In order to handle nested ControlFlowOps, it's most efficient to call the drawer class recursively. But only a few of the drawer methods are needed. Therefore for simplicity is was necessary to remove some attributes, such as _data and _wire_map and initiate these variables in the draw method and pass them to subsequent methods. The majority of the changes are these variable changes.
• Layer widths are now stored on a per-node basis instead of a layer number basis. This is necessary for the nodes in the recursive calls as the layer numbering is reset at each recursion.

These changes have been tested with all the existing ipynb image tests and the functionality of the drawer should be unchanged from the current main mpl drawer.

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[coveralls]

Pull Request Test Coverage Report for Build 5137779293

• 127 of 206 (61.65%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 85.933%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/visualization/circuit/matplotlib.py	127	206	61.65%

Totals
Change from base Build 5127277220:	0.02%
Covered Lines:	71292
Relevant Lines:	82962

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💛 - Coveralls

[jakelishman]

If I understand correctly, most of this is about removing some of the global state from MatplotlibDrawer, and passing it a little more explicitly around in the (to be added) recursive calls? I'm totally on board with that.

Removing the Anchor class also seems fine to me, and it's super promising that the tests didn't need any modifications with this. Thanks Edwin!

[jakelishman]

Wait sorry, I just realised that the MPL tests don't run in the standard format (at least without #9961) - I'll probably merge this in a sec, I'll just check the outputs locally first.

[jakelishman]

I've verified locally that this PR introduces no changes to the visual test suite compared to main.