Create additional otu-table transformation methods

[mikemc]

Currently, all we have is transform_sample_counts, for applying a function to a vector of counts from an individual sample. However, sometimes we might want to transform counts by taxon, instead of by sample, or to apply sample-specific scalings. Currently these have to be done with apply and sweep on the otu matrix, which is a headache to get the orientation and axes correct, so a function that works on otu tables / phyloseq objects and abtracts out this detail would be useful