Reverse mapping algorithm from qubit space to fermionic operators

[mb6-31ansatz]

What should we add?

Hi, I would like to request the addition of the feature reverse the mapping procedure from the qubit space. At the moment it is possible only to map to the qubit space.

[mrossinek]

#255 already suggested this specifically for the Jordan-Wigner mapper. If this is something you are interested in working on, that might be a good place to start 👍