comment fixes
Fix for boostrapping bug reported by @nhoffman

The float part of the namlom was not being taken into account.
Fixed Placerun_io to not fail with no placements.

Previously, writing out a placefile containing no placements would fail.
Allowing multiclass in add_classifications.

Also updating the 'seq' field to be called 'seqname' in line with what
taxtastic already uses.
Sorting classifications as they're added.

This will effectively order by the new rows in multiclass by tax_id, which
seems to affect how multiclass_concat creates synthetic taxa.
Adding initial support for all-rank NBC.
Merge remote-tracking branch 'local/multirank-nbc' into 290-nbc-ties
Fixing seeded NBC support.
Merge remote-tracking branch 'local/multirank-nbc' into 290-nbc-ties
Allowing multiclassification to be disabled.

If the provided multiclass-min is less than 0, multiclassification is disabled.
Merge remote-tracking branch 'local/multirank-nbc' into 290-nbc-ties
Merge branch '290-nbc-ties' of github.com:matsen/pplacer into 290-nbc-ties

Conflicts:
	pplacer_src/nbc.mli