output of measurement_outcomes should be serializable

[vishal-ph]

Prerequisites

Before raising this issue, I have already checked that I am:

• running the latest version
• made sure that this issue has not already been filed

Describe the bug

Running a QAOA computation on a QPU and attempting to dump the results fails because some QPUs return their measurement_outcome probabilities as a numpy.float, which is not python serializable. This issue may also happen for statevector simulators, where the measurement_outcomes are wavefunctions which are complex arrays.

To Reproduce

Steps to reproduce the behavior:

• Run a QAOA computation on a QPU or a statevector simulator
• Attempt to dump the QAOA results in a JSON file

Expected behavior

OpenQAOA should convert all non-serializable outputs to native python types, so that QPU results including measurement outcomes can be dumped into a JSON