Helper functions for reproducing statistical modeling results that can be applied to subsets of the available data or new degradation datasets

[lcolladotor]

We will write helper functions for reproducing the "main" and "interaction" model results based on the data provided in #32. These functions can then be applied to either subsets of the data to compute new statistical results that users might want to use as input (related to #33). Alternatively these functions could be applied to new degradation data from either new experiments carried at LIBD or data generated elsewhere for other tissues / organisms.