IOC: McLennan homing behaviour in internal mode

As an HRPD instrument scientist my McLennan when in internal homing mode (0) does not set the position to 0 and I have to remember to run a script `inst.zero_mclennan_after_home` I don't want to have to do this.

We need two modes:

1. Go to forward limit and home back to switch
2. Go to reverse limit and home forward to home switch

Acceptance criteria:

1. Pressing home (reverse or forward) initiates a home in which:
1. a constant velocity jog to the limit switch is performed
1. a home command is issued
1. offset is read
1. when homed dial coordinates are set to 0
1. offset is reapplied
1. As 1 but with other mode
1. Function is removed from the script