Get grid scan validity check PV into vmxm FGS IOC

[olliesilvester]

VMXm's FGS IOC currently doesn't include a PV which i03 and i04 has. This boolean PV should look at the FGS parameters and validate them against the sample stage motor limits. It isn't as crucial for VMXm to have this check, since the grid scan will still be triggered through GDA, which has its own checks, but it's still I useful thing to have

I have asked controls to add this PV already

Acceptance Criteria

• PV added, tested and deployed