Fixed: horizontal interval calculations with FilterByOffsetIndexer

Horizontal intervals were incorrectly repeated when the lowest
voice was sustained longer than the offset interval. Fixed by setting
is_horizontal kwarg in the WorkflowManager.

Related to github issue #305
Tests: added integration test for previous FilterByOffsetIndexer bug

Tests proper calculation of horizontal intervals and adds a new piece
to the test corpus, Kyrie_short.krn.

Closes #305