One to One Track Association

[gawebb-dstl]

I've implemented a different track to track association algorthim using the 'assign2D' method in stonesoup.dataassociator._assignment. This track to track association only allows tracks to be associated on a one to one basis, so tracks can't be associated to multiple other tracks.

The algorithm itself is pretty arbitary and can/should be improved; it just looks back over the last 10 states in a track and applies a distance measure to those states