Rework fastener.head_height

[Roger Maitland]

Change the head_height algorithm from this:

    @property
    def head_height(self):
        """Calculate the maximum height of the head"""
        if self.head is None:
            result = 0
        else:
            result = self.head.vertices(">Z").val().Z - self.head.vertices("<Z").val().Z
        return result

to one that uses a bounding box.