Conversion of openfermion QubitOperator to PauliStrings operator in netket
Created by: jwnys
openfermion has good symbolic operators that can complement netket. E.g. one can define FermionOperators and convert to QubitOperators via jordan_wigner transformations. If we convert the symbolic openfermion.ops.QubitOperators to nk.operator.PauliStrings, we cover quite a lot already. This can help e.g. in reproducing "https://www.nature.com/articles/s41467-020-15724-9" easily (good idea for an example/tutorial?) I'm adding it already, but it'll need an update when PR #955 is merged. This will improve testing as well.