Move to specified chis and move aperture/backlight during robot load and centre

[DominicOram]

A common recipe in UDC is:

1. Load pin
2. Do XRC + rotation at chi = 0
3. Do XRC + rotation at chi = 30

GDA currently treats these as two collections and will try and get GDA to load the pin again in between 2 and 3. Hence the requirement for #240. However, we also need to do the move to whatever chi/phi is specified before doing the XRC and make sure we're in the correct position for snapshots (as we can't just assume it that we were left in it after robot load). This may be more cleanly solved by #470 but for the time being we should make that when calling a robot load and centre we are:

• Moving to the correct chi/phi
• Moving the backlight in and aperture out
  Before doing XRC. It probably makes sense to do this in the centring plan but we should also make sure that if we're already in the correct state it doesn't add time

Acceptance Criteria

• Before doing a centre we make sure we're at the right chi/phi and that the backlight is in and aperture out
• If these things are already in the right position no extra time is added