Computing generic observables

[agata-branczyk]

Is your feature request related to a problem? Please describe.
It would be useful to be able to compute generic observables.

Describe the solution you'd like
The ability to evaluate generic operators of the form X = \sum_\mu A_\mu \otimes B_\mu where A_\mu and B_\mu are weighted Pauli operators acting on the first and second halves of the partition respectively.

Additional context
Some examples of observables that might be interesting are 1-body and 2-body matrices.

The 1-body matrices are used for:

• electrostatic properties
• electronic densities
• molecular electrostatic potential

2-body matrices are used for:

• orbital optimization
• analysis of correlation functions

The combination of both is used in entanglement analysis.