Keep Moving Average of Metrics

[dbent]

The changes in KSP 1.0.3 made a lot of metrics change their value rapidly from frame-to-frame. This isn't terrible for the context menu, but causes overlays to appear to flash with different colors as the gradient color values change.

To alleviate this we should keep a moving average of the various metrics over some configurable number of frames. Overlays should use the moving average values by default while the context menu should use the "instantaneous" value by default. Whether to use the average or instantaneous values should be configurable.