Fine tune summary legends

[mortonjt]

It should be possible to retrieve the input strings for the --p-formula option for each regression model from the qiime2 providence. Would be super useful if these formulas could be directly displayed in the summary. For example in the image below, rather than displaying model vs baseline, it would display "Depth+Temperature+Salinity+Oxygen+Fluorescence+Nitrate" vs "1" (using the formulas from the red sea tutorial)