add in mass migration event

[GertjanBisschop]

As pointed out by @A-J-F-Mackintosh, the currently available Event classes could be made more flexible by adding in a discrete MassMigrationEvent that moves all lineages from one population to another at a discrete point in time.

[GertjanBisschop]

General pattern of _single_step(): as you said in #20 @A-J-F-Mackintosh, this method returns the index of the event, and the new configuration of lineages after a single step. The count essentially tallies the number of ways that the same new configuration can be achieved by combining different lineages. For example a pairwise coalescence event for ['a', 'a', 'a'] will always result in ['aa', 'a']. However, there are 3 different ways of achieving this result.