Require that Q-planes have strictly increasing frequency

Created by: alurban

This PR enforces monotonic-increasing frequency in all QPlane objects by stripping off duplicate frequencies, while preserving the order. This resolves an issue where duplicate frequencies cause scipy.interpolate.interp2d to return all NaNs, for a demonstration see this omega scan (requires LIGO.ORG credentials).

An extra unit test checking monotonicity is included.

This fixes #1155 (closed).

cc @duncanmmacleod, @dethodav