WIP preconditioned implementation of NEB and String MEP search methods

This MR implements preconditoned variants of the NEB and String methods, as described in the following article:

S. Makri, C. Ortner and J. R. Kermode, J. Chem. Phys. 150, 094109 (2019) https://dx.doi.org/10.1063/1.5064465