Cleaning the code base

[Mayank447]

• Removed unused import completely and made imports absolute (i.e. qutip_qip.circuit import QubitCircuit) instead of relative ..circuit import QubitCircuit. This helps when files are moved around or refactored.
• Removed many of the undefined variables, there are still a few left. Those will be removed in subsequent PRs as there are deeper errors in the code.
• Unused variables removed, similar point as above.
• Code refactored: qasm.py has been broken down into a module since the no. of lines of code was over 1300 (the external import structure remains the same), separated Model into a separate file, renderers were refactored into a module name draw under src/circuits

@BoxiLi This PR isn't fully finished yet, its still a draft. But can you review it please since the code changes especially refactor is pretty huge.

[BoxiLi]

Nice thanks for doing this. I will have a look, but try to avoid exploding the PR. Let's keep the size as small as possible (At the moment it seems ok, maybe move the qasm and circuit drawer change to a different one.). I just noticed that in the last PR the test_qiskit was mistakely commented out in the end, which I missed.

Also to avoid having to manually slient the qiskit test, I would recomment to create a different branch instead of using master and create a new environment, either using conda or virtual env. So the development won't affect your existing environment. And you can have multiple version at the sametime. This is good practice in general.

[Mayank447]

Sure, I will keep this in mind. I will move qasm, circuit to a different PR, those refactor are pretty huge.