<unset> removes struct entries with inheritance again

Giving the <unset> key when inheriting ifos removes that entry from the inherited struct. For example, the arm power can be set directly with

+inherit: 'Aplus'

Laser:
    Power: <unset>
    ArmPower: 730e3

Laser.Power needs to be removed from the struct because that cannot be simultaneously defined with Laser.ArmPower.

A test was also added to test this behavior.