refactor for improved testing of parameter settings #182
Description
We should refactor to enable easier testing of parameter settings. Specifically, it would be good to be able to test that parameters passed through the QIIME 2 action make their way into the R code. This could be achieved by having a private function that constructs and returns the command. We could then test that the command that would be called has the parameters set as expected.
The approach for achieving this will depend on our ultimate plan for #172.