Test that tests based on EmpiricalPotentials

[mfherbst]

Once EmpiricalPotentials is ready replace the current dummy test by a test based on a LJ or similar, which actually tests the geometry optimisation does its job. In particular test:

• The energy decreases due to the optimisation
• The correct minimum is obtained
• Custom callbacks
• Other Optimization.jl solvers

[mfherbst]

Could also use a simple calculator from ASE for now, see https://mfherbst.github.io/ASEconvert.jl/stable/#Employing-ASE-calculators-in-Julia