WIP: Mrchem+DMRG

[martinanibbi]

Work in progress: Integration of DMRG in MRChem.

[gitpeterwind]

I have made a two electron integral code in #532 , so that we can write:

#include "qmoperators/two_electron/two_electron_utils.h"

void ExternalSolver::set_two_body_integrals(OrbitalVector &Phi, GenericTwoOrbitalsOperator &g) {
    this->two_body_integrals = std::make_shared<ComplexTensorR4>(calc_2elintegrals(prec, Phi));
}

(How can I propose my merged branch directly as a pr into your pr?)

[martinanibbi]

Sorry for the late reply, I think you should send a PR to the forked MRChem here: https://github.com/martinanibbi/mrchem/tree/mrchem-dmrg
Then it should automatically appear here.