Efficient batched circuits in qiskit

[lazyoracle]

What

This PR reworks parts of the qiskit interface to allow more efficient batched circuit execution. Qiskit jobs can contain multiple experiments (each experiment containing a qasm circuit) and these are now batch processed without recomputing expensive propagators.

Why

Closes #199

How

C3 experiment object is now initialised at the job level, and not at the qiskit experiment level. Propagators are also created once per job and not for every experiment in a job. If a particular experiment has a SetParamsGate the propagators are recomputed.

Remarks

Check the Full_Loop_single_qubit.ipynb for an example of processing batched circuits.

Checklist

Please include and complete the following checklist. Your Pull Request is (in most cases) not ready for review until the following have been completed. You can create a draft PR while you are still completing the checklist. Check the Contribution Guidelines for more details. You can mark an item as complete with the - [x] prefix

• [x] Tests - Added unit tests for new code, regression tests for bugs and updated the integration tests if required
• [x] Formatting & Linting - black and flake8 have been used to ensure styling guidelines are met
• [x] Type Annotations - All new code has been type annotated in the function signatures using type hints
• [x] Docstrings - Docstrings have been provided for functions in the numpydoc style
• [ ] Documentation - The tutorial style documentation has been updated to explain changes & new features
• [x] Notebooks - Example notebooks have been updated to incorporate changes and new features
• [ ] Changelog - A short note on this PR has been added to the Upcoming Release section

[codecov[bot]]

Codecov Report

Merging #217 (f0d16d7) into dev (ad9ba29) will decrease coverage by 0.72%. The diff coverage is 90.00%.

@@            Coverage Diff             @@
##              dev     #217      +/-   ##
==========================================
- Coverage   75.02%   74.30%   -0.73%     
==========================================
  Files          38       38              
  Lines        5593     5545      -48     
==========================================
- Hits         4196     4120      -76     
- Misses       1397     1425      +28     

Impacted Files	Coverage Δ
c3/qiskit/c3_backend.py	90.90% <88.88%> (-2.85%)	:arrow_down:
c3/qiskit/c3_provider.py	85.29% <100.00%> (ø)
c3/qiskit/c3_backend_utils.py	26.78% <0.00%> (-33.93%)	:arrow_down:
c3/experiment.py	74.83% <0.00%> (-2.32%)	:arrow_down:

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