Reenable tree confidence intervals
Reenable tree confidence intervals

Previously we checked for the presence of num_date confidence on the
first node (nodes[0]). Prior to auspice handling subtrees (arrays of
trees), nodes[0] was guaranteed to be the root node, but this is no
longer the case. Checking a single node wasn't the best approach anyway
as we should check for _any_ node with confidence set, which this
commit now does.

nextstrain/auspice,nextstrain/auspice,nextstrain/auspice