molar-volume

### What is the expected behavior? While **Jordan-Wigner transformation** is implemented in the `mappers` framework (`JordanWignerMapper`), the reverse transformation is missing. Inspiration can be drawn from https://github.com/quantumlib/OpenFermion/blob/master/src/openfermion/transforms/opconversions/reverse_jordan_wigner.py PS: If maintainers...