Allow construction of self-consistent dispersion

[awvwgk]

Is your feature request related to a problem? Please describe.
DFT-D4 can be used self-consistently together with tight binding methods. For this purpose the dispersion evaluation has to be tightly integrated in the potential evaluation of the SCF iterations.

Using self-consistent DFT-D4 in the standalone dftd4 binary or with the library is more complicated (cyclic dependencies) and probably out-of-scope for the main library.

Describe the solution you'd like
This projects provides the basic building blocks to create a ready-to-use self-consistent dispersion.

• creating of potential intermediates (caching of dispersion matrix, gaussian weights for references)
• potential evaluation (dE/dq, downstream is required to convert it to a Hamiltonian)
• self-consistent energy/gradient evaluation (split of ATM and self-consisten two-body evaluation)

Describe alternatives you've considered
Projects with self-consistent DFT-D4 maintain their own implementation (see DFTB+ and xtb).