Dev gate applicator

[shahidee44]

Description

• Changed the way Gate Objects are structured. Each non-core openqaoa module will need a GateApplicator object which contains library specific functions on how the gates are applied to the circuit. The Gate Object now acts as an interface.
• Each Backend now creates a GateApplicator Object that, together with the Gate Object apply the right library gate to the Backend's internal circuit
• The implementation of GateApplicator for Qiskit, Braket and Pyquil backends.
• Dynamic generation of constants: DEVICE_NAME_TO_OBJECT_MAPPER , DEVICE_ACCESS_OBJECT_MAPPER. Based on the presence of the particular module.
• get_qaoa_backend and create_device updated to dynamically find a module if it has been installed.
• plugin_finder.py is added which helps facilitate the function needed to dynamically find modules within the environment.
• Updated test_gates.py unittests
• Added test_gate_applicator_qiskit.py unittests
• Added test_gate_applicator_braket.py unittests
• Added test_gate_applicator_pyquil.py unittests
• Added test_gate_applicator_vectorized.py unittests
• Edited setup.py to include entry_points which will be used by plugins to interact with core code.
• Removed core code dependency on any plugin

Checklist

• I have performed a self-review of my code.
• I have commented my code and used numpy-style docstrings
• I have made corresponding updates to the documentation.
• My changes generate no new warnings
• I have added/updated tests to make sure bugfix/feature works.
• New and existing unit tests pass locally with my changes.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Name the new unit-tests that you have added along with this change.

run all tests.

[codecov]

Codecov Report

Merging #193 (cfe711d) into dev (985ad99) will decrease coverage by 0.10%.
The diff coverage is 96.85%.

@@            Coverage Diff             @@
##              dev     #193      +/-   ##
==========================================
- Coverage   97.81%   97.72%   -0.10%     
==========================================
  Files          33       37       +4     
  Lines        6325     6769     +444     
==========================================
+ Hits         6187     6615     +428     
- Misses        138      154      +16     

Impacted Files	Coverage Δ
setup.py	100.00% <ø> (ø)
tests/test_backends.py	51.47% <0.00%> (-1.56%)	⬇️
tests/test_gate_applicators_vectorized.py	92.23% <92.23%> (ø)
tests/test_gate_applicators_braket.py	98.18% <98.18%> (ø)
tests/test_gate_applicators_pyquil.py	98.19% <98.19%> (ø)
tests/test_gate_applicators_qiskit.py	98.21% <98.21%> (ø)
tests/test_gates.py	100.00% <100.00%> (ø)

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[vishal-ph]

Since, I proposed the design in this PR, it makes sense that either @raulconchello, @Q-lds, be the primary reviewer of this PR. I am adding you both for now. I will be reviewing it too

[vishal-ph]

NOTE: If not done yet, run black formatter before merging