Allows for the calculation of coupled elecron-photon problems
by applying the dressed orbital approach. Details can be found in
https://arxiv.org/abs/1812.05562
At the moment, N electrons in d (<=3) spatial dimensions, coupled
to one photon mode can be described. The photon mode is included by
raising the orbital dimension to d+1 and changing the particle interaction
kernel and the local potential, where the former is included automatically,
but the latter needs to by added by hand as a user_defined_potential!
Coordinate 1-d: electron; coordinate d+1: photon.