Move the deme expander out of the metapop.

It appears that the metapop and the deme expander have nothing in common.
remove another metapop argument
port over the example progs to the new interface.
Split up metapop params struct into two
Move the deme expasion stuff to its own directory

It appears to sharre no commmon code or mechanisms with the
metapopulation, so move it out of the way.