Hessian Prototype

[cortner]

This PR is to provide a prototype hessian implementation. This is needed for a different project, so I can't wait around anymore until the community decides on the AtomsCalculators format.

Needs the FD PR in AtomsCalculators to be merged first.

• [x] fix ad_site_hessian
• [x] tests for site hessian (prototype)
• [x] implement global hessian assembly, both block-format and blas format
• [x] test global hessian assembly
• [x] sort out units

Comments

• This PR also changes the SW implementation slightly. SW now simply fails if the input structure is not elemental Si. We can debate this and I can reverse this change if I see a good reason. Personally, I don't see how SW can ever be used for anything other than elemental Si
• The hessian currently returns a dense format. This is obviously not ideal. I think of this implementation only as a prototype to be improved upon in a few more PRs.

[cortner]

this is ready for a review. ~Units still need to be taken care of.~

• Working with units in this framework is driving me to insanity. It still feels like a hack. We need to revisit this please. But not in this PR. We can hack it here.
• I would appreciate help with the tests. I keep getting this:

  (EmpiricalPotentials) pkg> test
   Testing EmpiricalPotentials
  ERROR: can not merge projects

[cortner]

Tests only fail because other PRs need to be merged. Locally all tests pass.

[cortner]

this is now superceded by #22 - but some of the tests still need to be moved over.

[cortner]

those tests are currently in ACU - not clear yet where they should actually live, but I think the PR can now be closed.