Fix: Mutations: proportion filter on grid view

[JonasKellerer]

#127 refactored the mutations component but introduced an unwanted behavior:
On the grid view, the proportion selector should filter out only those mutations, where non of the mutations on a given position are above/below the threshold. If only one is above, the others should still show.

Fix this and introduce a test, so this does not occur again.

Example of the issue:
Here only one mutation shows at 2683

With a lower threshold two are shown