Fix fourbarlinkage (backport #1837)

[mergify]

Summary:

Fixed effort mapping for FourBarLinkageTransmission, related to #1746.

New equations for effort mapping meet the principles of conservation of energy.

Actuator to joint:

$$\tau_{j_1} = n_{j_1} n_{a_1} \tau_{a_1} + n_{a_2} \tau_{a_2}$$ $$\tau_{j_2} = n_{j_2} n_{a_2} \tau_{a_2}$$

Joint to actuator:

$$\tau_{a_1} = \frac{\tau_{j_1} - \tau_{j_2}/n_{j_2}} {n_{j_1} n_{a_1}}$$ $$\tau_{a_2} = \frac{\tau_{j_2}} {n_{j_2} n_{a_2}} ```<hr>This is an automatic backport of pull request #1837 done by [Mergify](https://mergify.com).$$