Added support for variable spring constants to NEB. Should consider a dynamic version like the one in Graemes paper too.
Added support for the "old" spring force calculation, from the previous NEB implementation in ASE. It does not give the exact same results currently.
Changed the spring force method is decided. Added the option to control it individually.
Updated the way the spring constants are stored to match the old NEB implementation of ASE.
Whitespace.