Pauli Decomposition for Calibration Matrix

[NicoRenaud]

Any Hermitian matrix can be decomposed as a sum of tensor product of Pauli matrices (https://quantumcomputing.stackexchange.com/questions/8725/can-arbitrary-matrices-be-decomposed-using-the-pauli-basis).

That can be useful to represent the matrix (A) of the linear system (Ax = b) as a sum of a large number of shallow quantum circuits. We need to implement this decomposition in qalcore to complement the UnitaryMatrix class (https://github.com/QuantumApplicationLab/qalcore/blob/main/qalcore/qiskit/vqls/numpy_unitary_matrices.py)

[NicoRenaud]

I have a notebook somewhere that would be a good start for the class