add torque readings for implicit actuator

[chaofiber]

This MR makes the following changes:

• Add torque readings for implicit actuators

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• I have added tests that prove my fix is effective or that my
  feature works
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[Mayankm96]

Hi, thanks a lot for this MR. Can we add a unit test to check if the torques we obtain from the implicit models are similar to those we see with explicit models when we use a simple PD actuator model. In principle, it should be close if dt is small.