Fixed off-by-1 bug in get_sigsqs(after_burn_in=T)

get_sigsqs(after_burn_in=T) was returning the sigsqs
for the last burn-in forest in addition to all after
burn-in forests.
Merge pull request #3 from moserware/master

Fixed off-by-1 bug in get_sigsqs(after_burn_in=T)