Update get_major_lineage_prevalence date handling

[mindoftea]

Per discussion on PR #87, the get_major_lineage_prevalence function needs to be changed to take into account the min_date and max_date for filtering. The function has been updated (see commit 9cee401) to take these arguments as appropriate, however the output is not yet correct and the logic needs to be changed (see lines 208-215 of util.py in particular). The intended behavior would be to select records between min_date and max_date if they are specified, or records up to ndays before the present (defaulting to 180) otherwise.