Unexpected handling of incomplete file sequences #22

If a series only has frames, say, 40-60, the sequence loader currently appears to behave in what I would say unexpected ways for frame ranges outside of this range. I think there are at least two sensible options:

• Load empty geometry when the current frame is not available (basically making the object not appear or disappear)
• "Freeze" geometry to the closest available frame.

I think maybe we should support both, maybe choosing the first option as a default and allow toggling through settings.

We should also handle the case when only intermittent frames are available, for example 40-50 and 60-70, or more generally 3, 5, 8, 12.