aggregate_to_level: Aggregate data to a fixed level
In jacobbien/trac: Tree-based Aggregation of Compositional Data

This function aggregates a data set to a fixed level of a taxonomy. For example, if aggregating to the Phylum level, it would sum up the columns of x corresponding to OTUs/ASVs in that Phylum. It also adjusts the tree information (in particular A, tree, and tax) for this new smaller problem.

1	aggregate_to_level(x, y, A, tax, level = 7, collapse = FALSE)

`x`	n by p data matrix of counts, where n is the number of samples, p is the number of OTUs/ASVs.
`y`	response vector of length n
`A`	p by (t_size-1) binary matrix giving tree structure (t_size is the total number of nodes and the -1 is because we do not include the root). This is created by the function `phylo_to_A`.
`tax`	a tax table, which is a p by number-of-levels matrix, where cell ij gives the name of the level j ancestor of OTU/ASV i.
`level`	which level are we aggregating to?
`collapse`	see `tax_table_to_phylo`