1.7.15 tip_glom: Drastically simplify improve tip_glom() function

It is now a function.
It now uses core R infrastructure for agglomerative clustering.
It can flexibly accept different methods, clustering functions.
Still needs testing. Some unit tests might fail incorrectly. Also needs
new examples.
1.7.15 tip_glom() now uses standard R clustering tools, and takes their arguments

merge_taxa() now merges OTUs to the OTU that has the largest count
total (across all samples, taxa_sums() )
tests have been revised/augmented to reflect this latter change as well.
Merge branch 'revise-tipglom'