Move to the current API of qiskit-nature

[fretchen]

Summary

This attempts to fix #87 and most likely also #88

Details and comments

However, we have to jump from the syntax of 0.4 to the syntax of 0.7 for qiskit-nature and this seems to be a fairly major shift. I think that I might have found most of the operators, but some problems and questions remain: I think that I might have found most of the operators, but some problems and questions remain:

• It would seem that SparseLabelOp is the new name of SecondQuantizedOp ? Not sure what has to be changed to make it work.
• The register_length seems to not exist anymore and had to be updated to num_spin_orbitals or num_spins.
• For the moment the tests still fail with 'Gate rlx has no defined generator'. No idea why.
• I think that we should also enable dependabot updates to avoid another massive version jump like this one in the future.

[CLAassistant]

All committers have signed the CLA.

[fretchen]

@eggerdj I think that the comments should now be taken into account and we are again ready for review.