Refactor global optimizer

[gcattan]

pyRiemann-qiskit distance and mean methods take an additional parameter "optimizer", as compared to pyRiemann.

For these methods to be used within the pyRiemann MDM, we allow these methods to retrieve the "optimizer" from the global scope.

We can inherite from MDM inside pyRiemann-qiskit and change the signature of the methods used inside the MDM.