sparse actuator_moment

[thowell]

sparse implementation for actuator_moment as part of the effort to implement sparse Jacobians #88

notes:

• unlike mujoco where moment_rowadr is computed dynamically by mj_transmission, this implementation statically computes moment_rowadr ahead of time in io.put_model and stores this field in Model.
• actuators including slidercrank, tendon, site, or body transmissions are currently assumed to have dense Jacobian rows.
• in the future it might makes sense to investigate dynamically computing moment_rowadr and storing it in Data if we end up trying to optimize scenes with transmissions mentioned above.
• this pr does not implement sparsity for body transmissions -- probably makes sense to wait until sparse efc_J #934 is merged before adding this feature since this transmission type depends on efc_J
• there are probably remaining performance optimizations for some of the sparse routine kernels and update kernels