New MainLocations for Step and Seat Lower Cross Member

[derpmann]

Especially for bigger cars we need a new MainLocation for the Step (german: "Trittstufe").
I propose the name STEP.
The Figure VEH_1 has to be actualized.

In addition we need a new MainLocation for measurements at the Seat Lower Cross Member (german: "Sitzquerträger").
I propose the name SLCM.
The Figure INT_1 has to be actualized.

[Admin]

integrated into database and figure for v1.6.
SLCM is part of the seat; SECM added for the cross-member on the vehicle body.