Calculate back lug position from leg_r radius not the stage height

From #393 (closed), the main body does not scale properly when changing the height.

This MR redefines the back lug position so that it does not move when the stage height is changed, but does move when the stage radius is changed.
The lug actually moves from the previous standard by 0.106mm in this MR, as noted in #393 (closed).

Note here as it was the motivation for changing height:
To change the stage xy range of motion, while keeping the actuator position and actuator thread range of motion the same, both the stage height, sample_z, and the xy_lever_ratio need to change in default_params():
The actual leg height is less than the sample_z, by stage_t, which is 15mm. So to change from the 'standard' 12mm range of motion to Xmm sample_z goes from 75mm to ((75-15) * (X/12)) + 15 and xy_lever_ratio goes from (4.0/7.0) to (4.0/7.0) / (X/12).

closes: #393 (closed)