Added a popularity ratio filter to filter on favorites per find.

This complements my pull requests  #3385 and #3389 where I proposed a comparator to sort on popularity ratio and a filter to filter on pure popularity.  With this pull request I propose a filter to filter on popularity ratio as well.

As a result of these three pull requests a user can now 
- sort and filter on pure popularity (favorite count)
- sort and filter on popularity ratio (favorites per find)

Furthermore, I removed the constraint that only caches > 9 finds are handled in the popularity ratio sorts and filters. Showing it to potential users they rather got confused by this not apparent tweaking of the statistics, when a cache with 8 finds and 2 favorites did not appear in the >20% filter. As a consequence, now a cache with a single find and a single favorite (having a ratio of 100%) will lead a sort and pass the >75% filter even though it may not be very attractive due to the low number of ratings. This however, is then obvious to the user since he clearly can see in the cache list that this cache has only a single favorite.