Change Newick validation from `sniff` to `validate`

[ChrisKeefe]

Improvement Description
Proper validation, possibly including empty-tree detection, could better protect users from passing bad phylogenies.

Current Behavior
NewickFormat uses a sniff pulled from skbio. If this is the sniffer linked in References, it catches empty trees, but only checks 100 tokens.

Proposed Behavior
A more comprehensive validation.

Questions

1. Is the skbio sniffer already doing what we need?
2. Is a full-length validation reasonable, timewise?

References

1. This might be our sniffer?
2. Newick overview

[nbokulich]

Is the skbio sniffer already doing what we need?

In this same vein, I wonder if the validator should also check that all branches have a length.

This error sometimes crops up on the forum, e.g., when a tree is imported from outside (including some reference phylogenies!)