Energy changes: Allow extrapolation for pitch

[DominicOram]

When going to very low energies we need to set the DCM pitch to a value outside that in the lookup table. GDA will extrapolate the last two values in the lookup table and use these to extrapolate outside its range. Currently hyperion refuses to extrapolate but we probably have to.

Acceptance Criteria

• Work out the best method of extrapolation
• Implement in hyperion

[DominicOram]

If it's below the last energy use the last value.