[feat] GeneralBraOpKet and mutable tensors in GeneralState

[PabloAndresCQ]

Description

• Refactor of GeneralState so that it uses CuTensorNet's NetworkState
• Support for mutable tensors on GeneralState, so that parameterised circuits can be simulated without optimising the contraction path again for different parameter values.
• Includes a new GeneralBraOpKet operator that can be used to calculate expectation values, inner products and arbitrary matrix elements.
• Some misc improvements to the general_state module here and there.

Related issues

#150

Checklist

• Write a notebook tutorial.
• I have performed a self-review of my code.
• I have commented hard-to-understand parts of my code.
• I have made corresponding changes to the public API documentation.
• I have added tests that prove my fix is effective or that my feature works.
• I have updated the changelog with any user-facing changes.