Make TreePrior.add_state_nodes use state instead of beastxml

[Anaphory]

This line

BEASTling/beastling/treepriors/base.py

Line 42 in ba26491

self.add_tip_heights(beastxml.config.tip_calibrations)

is the only one in TreePrior.add_state_nodes that makes use of the argument beastxml, instead of its beastxml.state attribute, and for a frivolous purpose. We should make the tree prior generally aware of tip calibrations (so eg. the Yule Prior can insist that there be none), and then make add_state_nodes only aware of the state to put its own state nodes into.