Extract GMLS solution from class

[kuberry]

GMLS solution (alphas) are currently stored inside of the GMLS class.

This should be extracted to it's own class (GMLSSolution) which can be fed to an evaluator class or queried similarly to how GMLS class is currently queried for the solution.

This would reduce complexity of GMLS class and enable merging multiple GMLS solutions.
A helper could be created that would merge multiple GMLS solutions along with corresponding NeighborLists classes (checking for consistency), and this would allow for dynamic updating of lists, appending by on a few or a single additional target site.

This would be useful in addressing #52