Replace lazy loading of vectorize_counts_and_tree

[RMeneses99]

Problem

_vectorize_counts_and_tree was a private scikit-bio API that was promoted
to public as vectorize_counts_and_tree in scikit-bio >= 0.6. The top-level
import of the old private name broke all gemelli imports on modern scikit-bio
versions, even for users who never use phylogenetic methods.

Fix

• Removed the broken top-level import
• Added a lazy import inside fast_unifrac() where it is actually used
• Updated the call site to use the public name

Fixes #112

[cameronmartino]

@RMeneses99 just realized you fixed this before me! Thank you!!! Fixed in the other PR. Much appreciated!