ADF: Transition State searches: partial Hessian and improved NEB

A partial Hessian can be calculated, which is fast to calculate for large molecules compared to the full Hessian. This means that the 'active atoms' can be treated exactly, the rest of the Hessian can be calculated with a semi-empirical estimate.

The Nudged-Elastic-Band (NEB) implementation has also been improved and should converge in fewer steps now.