Add Snapshot support for numerical node label interpretation

[jcoliver]

When a file includes trees with node labels, Mesquite prompts the user to indicate how the values are to be interpreted (e.g. bootstrap frequency, consensus frequency, posterior probability, etc.). The querying is through ManageTrees.queryAboutNumericalLabelInterpretation, which only ever appears to be called by MesquiteTree.checkNumericalLabelInterpretation.

So even when running Mesquite scripts with a file that has trees with node labels, a dialog opens. If this information could be included via a Snapshot object (thus circumventing dialogs), it would add useful functionality. Not sure which module would be responsible for writing & reading the Snapshot, though. ManageTrees doesn't quite operate like ManageTaxa...