Optimise assignment of colours

[jack89roberts]

The use case of distinctipy is that you need N distinct colours for N different groups in some kind of visualisation. The part of picking N colours is implemented, but it would be nice to also try to optimise which colour should be assigned to which group. E.g. Given co-ordinates for each group/the members of each group, find a colour assignment that maximises the colour distinction between the group and its neighbours.