Use map for CoordSets

[Tom Goddard]

For performance, change CoordSets from vector to map. As per T.G.:

I think finding a coordinate set from its id needs to be a map lookup rather than scanning the vector of potentially thousands of coordinate sets. For instance if I cycle through every coordinate set (for instance to compute crosslink lengths) then I end up looking up every single coordinate set id and with a vector scan it takes order N2 time where N might be 10,000 coordinate frames. It’s not a problem with my current 1000 coordinate set ensembles.