Consider interpolator passing verbatim non-number parameters rather than disallowing them

[bdowning]

This might allow for more efficient preprocessing of data that either doesn't change or uses a 'step' easing (i.e. doesn't change at all until the next keyframe, then snaps to the new value); you could skip tearing apart and rebuilding strings this way.

Needs thinking about because of the chance for different state signatures between keyframes, as this might cause trouble.