FormStrippingStrength should be in Pset_ConcreteElementGeneral

I've recently learned that FormStrippingStrength is not specific to precast elements (it is currently found in Pset_PrecastConcreteElementGeneral) so I propose to move it to Pset_ConcreteElementGeneral so it applies to all concrete things, precast or not.

Ping @Jesusbill @jmirtsch

Review the IFC property-set definitions for Pset_PrecastConcreteElementGeneral and Pset_ConcreteElementGeneral. Confirm where FormStrippingStrength is currently defined and update the specification so it applies to general concrete elements rather than only precast elements. Done means the property is removed from the precast set and present in the general concrete set.

Written by the indexing model from the issue text.