Completed Scara-calculations

update_current_position: I'm not sure if this implementation is complete
for scara, because it has no reference to the home-position/offset, just
the tower/pole-offset. Based on Quentin Harley's implementation, I
corrected the forward kinematics, becasue Quentin Harley's
implementation of forward kinematics seems to miss something, while his
inverse kinematics do look correct.