`RenumberTips.numeric()` on integer-labelled trees

[ms609]

With
RenumberTips(PectinateTree(11:1), BalancedTree(1:11))

the tipOrder is interpreted as a numeric, rather than as identifying tip labels.

Determine the desired behaviour here: perhaps a warning; perhaps coercing to character when TipLabels(tree1) is numeric?

Then implement and document