Create function to facilitate performing calibration from a reference species

[mikemc]

It might be convenient to have a function that performs a simple plug-in approach to the the 'reference-species' approach to calibration described by https://github.com/mikemc/differential-abundance-theory. In its simplest form, the function simply needs to take an 'observed' matrix and a set of reference measurements for 1 or more species, and it can calibrate all species in observed by multiplying by the geometric mean of the reference measurements divided by the observed measurements for those species. However, since I don't necessarily recommend such a non-statistical approach except for exploration and demonstration, it might be better to instead just make a function that facilitates applying sample-specific normalizations - essentially, an easier to use version of 'sweep()'. This function would allow any type of normalization, including to the total abundance (as in so called 'quantitative microbiome profiling')