IMP: restructure dada2 invocation from R script to rpy2

[colinvwood]

Currently we use an R script (q2_dada2/assets/run_dada.R) to run the dada2 pipeline for all four denoise methods in this plugin. Thus separate logic for each of these actions exists in each of the python functions and is also intermingled in the R script. Instead we should use the rpy2 library to interact with dada2's API within each method, increasing modularity. This will also make the plugin more maintainable because our team is more familiar with python than with R.

[colinvwood]

This should probably wait until after #169 is merged.