rename tests
remove metadata method from AggregatedPieces
Merge remote-tracking branch 'origin/visX-devel' into visX-devel
change AggregatedPieces to have instance variable `experiments`, rather than an unnecessary accessor method.
Merge remote-tracking branch 'origin/visX-devel' into visX-devel
simplifications

- IndexedPiece.indexers_used can just be accessed as a normal instance variable
- IndexedPiece._import_score implementation uses public metadata method, rather than accessing the private _metadata variable.
refactor IndexedPiece tests

there will be one test_* method per method in the class being tested.
text formatting

ocpd need to have no space before the colon.
extra TODO

important to note that this doctest does not work.
fix broken references to `mpc` in Indexer