example updated

COMPASS.R
added two filtering criteria
@param filter_lowest_frequency is a numeric that specifies how many of the lowest frequency expressed markers should be removed.
@param filter_specific_markers is like filter_lowest_frequency but lets you explicitly exclue markers. These filters help lower the number of singletons.

Bug fix to account for cells in excluded categories. These needed to be added to the null category. i.e. if you remove a category with 1 cell in each subject, that cell has to be added to the null category so that the n_s vector per subject matches the total counts per subject.

Bunch of other small changes related to filtering markers.

Return of the discrete model is wrapped in a list with components fit and data. Data is the ns and nu counts tables. Useful for comparing the empirical proportions. We can change this later.