Add support for previous optimisations

[oliverfleetwood]

Optimizations present in my PLUMED version of the code.

• Distribute the points on the string to maximize transitions. Much support exists already. You need to compute the arclength_weights and pass that as an argument to the string reparametrization method.
• Increase the number of points on the string at a fixed interval
• Coordinate exchange between points: pick the output coordinates closest to the new restrained point instead of always extending the previous restrained simulation. This will reduce the effect of orthogonal degrees of freedom.
• Adaptive number of swarms (lower prio).