Fix filtering final results by center and station.

When filtering final results by center and station id's, all center results get filtered out without taking into account the selected stations.

[x] Update the results_queryset function to take into account the selected stations. The logic can be found inside this if block.
- [x] Use the get_centers_stations function to get stations id for the selected centers then check if they exists in the stations id's that have been selected.
- [x] If True, filter queryset by the selected stations id's alone and if False filter queryset by the selected center id's alone.

onaio / tally-ho