generalize regression approach

[knaaptime]

currently, the function that does the heavy lifting in the regression approach is hardcoded to use pixel counts, but that precludes the ability to specify more complex models (e.g. log-transforming the pixel counts, adding interactions, or including more input data).

instead, we should allow the function to take a patsy-style model formulation as a string instead of assuming we know what the user wants in this line

[knaaptime]

resolved by #39