Make use of SpawnHeightOffset for landed pads

A quick addition... I had a few difficulties with the plugin when using it to build a return vehicle from a sea level base on Eve. The insane dV required to get into orbit requires a mammoth craft, and with the vehicle spawned in contact with the pad, it ends up crushing the pad when the vehicle is released. Launch stabilizers don't completely mediate the problem--super lifters will still sag once the physics has started. The fix is super easy, though... I added a bit of code to let the user choose the spawn height offset of the vehicle just before finalizing the build: anything from 0 to 10m is allowed, and only if the pad is landed. With this change a super heavy lifter can sway and sag happily a few meters above the pad without crushing it (or anything to which it's attached). One note--you may want to consider upping the maxTemp for launchpad parts--I'm using Module Manager to increase the value so my super heavy lifter won't incinerate the pad on liftoff.