Adds custom barrier gate with conversion to/from qasm

[Aaron-Robertson]

Description

Fixes #1391

License

• I license this contribution under the terms of the GNU GPL, version 3 and grant Unitary Fund the right to provide additional permissions as described in section 7 of the GNU GPL, version 3.

Before opening the PR, please ensure you have completed the following where appropriate.

• I added unit tests for new code.
• I used type hints in function signatures.
• I used Google-style docstrings for functions.
• I updated the documentation where relevant.
• Added myself / the copyright holder to the AUTHORS file

[github-actions]

👈 Launch a binder notebook on branch unitaryfund/mitiq/ar-barrier

[codecov]

Codecov Report

Merging #1830 (382ba33) into master (58dfd2d) will decrease coverage by 0.89%.
The diff coverage is 59.70%.

@@            Coverage Diff             @@
##           master    #1830      +/-   ##
==========================================
- Coverage   98.54%   97.65%   -0.89%     
==========================================
  Files          73       73              
  Lines        3357     3458     +101     
==========================================
+ Hits         3308     3377      +69     
- Misses         49       81      +32     

Impacted Files	Coverage Δ
mitiq/interface/mitiq_qiskit/conversions.py	86.04% <59.52%> (-12.87%)	⬇️
mitiq/utils.py	89.62% <60.00%> (-10.38%)	⬇️

... and 2 files with indirect coverage changes