Warn if node labels mismatched

[ms609]

Resolves #113

[stitam]

Many thanks @ms609. I am wondering if warning() is strong enough, I would probably fix an inconsistent phylo object before going further, just to be safe. Do you think stop() would be too much?

[ms609]

I went for warning() because there's no absolute necessity to abort – a user may have some obscure and unanticipated reason for wishing to override the sanity check and proceed with mismatched labels.

[emmanuelparadis]

Many thanks for this. Now merged and pushed a new version here with updated files.
Almost ready to submit v. 5.8 to CRAN!