Add barrier labels and display in 3 circuit drawers

[enavarro51]

Summary

Fixes #8428 Fixes #1298

Details and comments

This PR adds a label parameter to the Barrier class and displays the label at the top of the barrier in the 3 circuit drawers. The existing label on snapshot directives is now also displayed. The display in the 'latex' drawer may not be optimal, but there are limitations in qcircuit for displaying text outside boxes. The info is still readable and does not overwrite any gate info.

qc = QuantumCircuit(2)
qc.x(0)
qc.y(1)
qc.barrier()
qc.y(0)
qc.x(1)
qc.barrier(label="End Y/X")
qc.draw('text')

     ┌───┐ ░ ┌───┐ End Y/X 
q_0: ┤ X ├─░─┤ Y ├────░────
     ├───┤ ░ ├───┤    ░    
q_1: ┤ Y ├─░─┤ X ├────░────
     └───┘ ░ └───┘    ░    

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[coveralls]

Pull Request Test Coverage Report for Build 2890221517

• 13 of 21 (61.9%) changed or added relevant lines in 5 files are covered.
• 4 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.002%) to 84.051%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/visualization/matplotlib.py	0	8	0.0%
<!--	Total:	13	21

Files with Coverage Reduction	New Missed Lines	%
qiskit/extensions/simulator/snapshot.py	1	76.74%
qiskit/visualization/matplotlib.py	1	25.29%
qiskit/extensions/quantum_initializer/squ.py	2	79.78%
<!--	Total:	4

Totals
Change from base Build 2882958684:	0.002%
Covered Lines:	56322
Relevant Lines:	67009

---

💛 - Coveralls

[enavarro51]

I'll dig into the text problem. It's repeatable. Replacing the barrier piece with text should not change the connections.

(Edit) This relates to vertical_compression. It can be set by the user or it happens when there needs to be space for text to display between gates vertically. Turns out this code does this,

qc = QuantumCircuit(6)
qc.cx(1, 4)
qc.x(1)
qc.draw('text', vertical_compression="low")

q_0: ──────────
               
          ┌───┐
q_1: ──■──┤ X ├
       │  └───┘
               
q_2: ──────────
               
               
q_3: ──────────
               
     ┌─┴─┐     
q_4: ┤ X ├─────
     └───┘     
               
q_5: ──────────
               

So this is something that needs to be addressed for controlled gates with low_compression. I assume open a separate issue for this.

[enavarro51]

Also on the font size for 'mpl' drawer. There are 2 defined font sizes in the styles - fontsize - fs and subfontsize - sfs, which users can change by setting the style parameter. I thought I would need to use sfs for the barrier label to avoid interfering with a control label, but after testing it, I think fs will work fine. I'll make the change.

Done in https://github.com/Qiskit/qiskit-terra/pull/8440/commits/1a17e25056c74e66f6229975af0e86aaf24ef759.