Numerical differentiation procedure

[awvwgk]

Is your feature request related to a problem? Please describe.

Related to grimme-lab/xtb-python#26. Currently a number of programs do not support Hessian calculations, but not all of them actually implement analytical Hessians. Implementing the numerical differentiation for each of those programs, either in the harness or upstream would lead to much duplicated code.

Describe the solution you'd like

The most straight-forward way to implement Hessian support for all these programs is by adding a numerical differentiation procedure in QCEngine. This also allows to use QCEngine's existing and future capabilities for parallelisation.

Describe alternatives you've considered

Require the upstream program to deal with it?

Additional context

ASE does not require calculators to implement Hessians, but provides its own numerical differentiation, see https://wiki.fysik.dtu.dk/ase/ase/phonons.html