robinson_foulds with 'unrooted_trees' and 'correct_by_polytomy_size' should probably not correct the root?

[Gullumluvl]

Hi,

I am currently using the version Ete3 but the current robinson_foulds source code from this repository is identical:

ete/ete4/core/tree.pyx

Lines 1286 to 1288 in 0f986e0

	if correct_by_polytomy_size:
	corr1 = sum(len(n.children) - 2 for n in origin_t.traverse() if len(n.children) > 2)
	corr2 = sum(len(n.children) - 2 for n in target_t.traverse() if len(n.children) > 2)

I think that the root node should be handled specially in the polytomy size correction when using unrooted_trees=True.

Otherwise, despite having a tree that is fully bifurcating, except at the root because of the convention for unrooted trees, you get the following error:

TreeError: 'Both trees contain polytomies! Try expand_polytomies=True instead'

I think that an update could be to check that the root node degree is greater than 3 when unrooted_trees is activated.

Best,