Expand zero gravity save data

[K-Meech]

For #20

See extra info from David on the issue: #20 (comment)

• adds additional save data for zero gravity
• re-names image files to match pose names from spreadsheet
• At the time of our last meeting (when David said we could remove all sway measurements), I had already finished refactoring TrackerBuffers so it could handle both head position (x/y/z) and angle (roll/pitch/yaw) data. Even though this is no longer needed for this PR I thought I'd leave it in as:
  • It gives us flexibility to calculate velocities in any direction / angle (in case David wants to add these sorts of measures in future)
  • It means we can now store head poses in one buffer, and calculate any of the associated position or angle velocities. This means that e.g. with RocketLaunch we no longer need to store the same head pose data in two separate buffers to get the pitch and yaw velocities - we can store the data once and calculate both directly from there.
  • See what you think - this refactoring can also be removed, if it's not useful
• Save data docs are updated