Add a KBMOD results filter for matching "known objects" (#741)

* Add filter for known objects

* Modify init with known object filter

* Clean up comments and tests

* Lint fixes

* Refactored to KnownObjsMatcher and added filters

* More refactoring and renaming

* Separate match vs obs_valid filters and clean up comments.

* Update test names and documentaiton

* Format

* Revert local change

* Remove blank line

* Address small comments

* Fix time filter unit conversion and testing

* Make obs_ratio and min_obs function parameters

* Remove unneeded obs_match_ratio field

src/kbmod/filters/__init__.py

@@ -1,5 +1,6 @@
 from . import (
     clustering_filters,
+    known_object_filters,
     sigma_g_filter,
     stamp_filters,
 )