JH command does not behave the way I expect

[kyle-kingsbury]

If current z < 0.25", then z retracts to 0.25" before moving x and y to zero. If current z > 0.25", then z does not move before moving x and y to zero. This would work okay if z zero is set to the top of the material, you should never crash. If z 0 is set to the bottom of the material, and the material thickness is greater than 0.25", then the bit will crash into the material possibly breaking the bit and damaging the workpiece.

Instead of a hard coded 0.25", what if the tool retracted to the safe z height? In certain cases it would already be there, like if you hit stop and quit before the JH command the tool would be at safe z height.

[RobMackie]

Look at #986 before doing this.