WIP: first attempt at implementing the use of filtering of test variable level combinations - has several non-trivial test failures. However, it's going in as a way point.
Add more detail to filter documentation.
Correct two bugs in tests - one was caused by the filter logic disregarding the implicit permutations of child factories, the other was caused by incorrect dissection of an expected test variable level combination by the test's own combined filtering logic. Still have at least one other known bug in these tests pertaining to the generated filters potentially filtering out *all* test cases, but the tests expecting at least one. Will fix this soon.