Move c3.qiskit outside c3-toolset

[lazyoracle]

Is your feature request related to a problem? Please describe.
The qiskit interface is an independent interface that need not exist within the core c3-toolset. The development can be decoupled and would benefit from existing independently of the core codebase. This would also reduce the dependencies for c3-toolset

Describe the solution you'd like
A separate c3-qiskit repository that acts as a plugin to the c3-toolset, with possible backend support for various hardware control stacks as well.

• Move c3.qiskit to a c3-qiskit repository
• c3-qiskit imports c3-toolset and qiskit as dependencies and implements Qiskit Provider, Backend and Job classes.

To-Do/Status

• Create a c3-qiskit package that works as a drop-in replacement for the current c3.qiskit
• Add tests to c3-qiskit to check integration with c3-toolset
• Update c3-toolset to use c3_qiskit in place of c3.qiskit
• Ensure integration tests for c3_qiskit are present in c3-toolset

[lazyoracle]

Probably part of a larger restructure and rewrite drive and not within the scope of current development. Work on enhancing qiskit integration with c3-toolset eg support for the full physics simulator to continue in this core repo.