add sfs output
also add mutation to the initial tree
Merge branch 'master' into sfs
update test README, now everything works
relax self test for active nodes

to deal with a harmless situation where the local root moves down during
coalescence because of pruning.
fix node update after pruning
remove overly strict self checks
add command line option for SFS output
Allow models with split out no migration
Merge branch 'sfs'
Merge branch 'master' into jz