Various minor API changes

[mrossinek]

This issue summarizes a number of various minor API changes open for discussion:

• [x] consider storing n_subsystems rather than compute it on the fly (see also #34)
• [x] rename all variable/properties beginning with n_ to be num_ (this is more inline with Qiskit, e.g. num_qubits on circuits and operators)
• [x] revisit the naming of some public attributes e.g. RandomizedProjectionMeasurements.msmt_qc
• [x] revisit naming of some function arguments e.g. obs should be observable in BaseDUAL.get_omegas
• ~re-evaluate the private/public attribute distinction on a number of classes~

[mrossinek]

Extracting the last task to #45