Add posibility of extra filling and apply to Kondo problem

For the Kondo problem with pseudofermions: that requires something that we currently don't have. The idea to solve the problem is adding an auxiliary field of Abrikosov fermions. Additionally one has to include a Lagrange multiplier for this field to fix the number of pseudofermions. We currently have just one multiplier for the "real" fermionic field. We'd need to add another one somehow. After adding these elements we can explore or reproduce the Kondo problem with it.

Some references already in the channel: arXiv:1509.05769, arXiv:2207.05780, https://link.springer.com/article/10.1140/epjb/e2014-50007-7