Reduce experimental folder size

With fb97b1cf users can now record at higher resolutions.

This does have an obvious effect on file size. For example a 5 minute experiment with 1296x972 will be ~250Mb.

The vast majority of space is taken up by the three different image files (sm_raw.npy, sm_skeleton.npy and sm_binary.npy). In principle we could just save sm_raw.npy and the threshold value and skeleton points for each frame. This should save a ton of space.