Effort Mapping of FourBarLinkageTransmission

[qiayuanl]

Describe the bug
The effort mapping between the joint and actuator of transmission_interface::FourBarLinkageTransmission is wrong!

Velocity from actuator to joint:

Effort from actuator to joint:

Consider a simple example with all reduction ratios becoming one and zero offsets:
Actuator side:
$\dot{x_a}_1=1$, $\dot{x_a}_2=0$, and $\dot{\tau_a}_1=0$, $\dot{\tau_a}_2=1$
Joint side:
$\dot{x_j}_1=\dot{x_a}_1=1$, $\dot{x_j}_2=\dot{x_a}_2-\dot{x_a}_1=-1$, and $\dot{\tau_j}_1=0$, $\dot{\tau_j}_2=\dot{\tau_a}_2-\dot{\tau_a}_1=-1$
The energy is not conserved:
$|\dot{x_a}_1\times \dot{\tau_a}_1| + |\dot{x_a}_2\times \dot{\tau_a}_2| = 0+0 \neq = 0+1 = |\dot{x_j}_1\times \dot{\tau_j}_1| + |\dot{x_j}_2\times \dot{\tau_j}_2|$

The correct mapping derived by Jacobian is:
$\dot{\tau_j}_1= \dot{\tau_a}_1 + \dot{\tau_a}_2$ and $\dot{\tau_j}_2= \dot{\tau_a}_2$

Environment (please complete the following information):

• OS: Any
• Version Any

[christophfroehlich]

Could you please open a PR fixing this?