Fit cosinor model for one aspect of data. Called by main 'kronos' function.
fit_groupwise_model(data, group, time, period, verbose)
input data
A character string. Signifies which group will be assessed.
A string. Should be the column name containing the time values.
A numeric. The length of a period, in the same format as the time
parameter.
A boolean. Toggles whether to print diagnostic information while running. Useful for debugging errors on large data sets.