[WIP] Mujoco interface for mc-rtc

Add this to your mc-rtc configuration yaml:

MUJOCO:
  xmlModelPath: "<path_to_your_xml_model>"
  pdGainsPath: "<path_to_your_pdgains>"

Usage:

Assuming that you have mujoco installed under ${HOME}/.mujoco/mujoco200,

$ git clone git@github.com:rohanpsingh/mc_mujoco.git
$ cd mc_mujoco
$ mkdir build && cd build
$ cmake .. -DCMAKE_BUILD_TYPE=RelWithDebInfo -DMUJOCO_ROOT_DIR=$HOME/.mujoco/mujoco200
$ make

Then, to run the interface:

$ cd mc_mujoco/build/
$ ./src/mc_mujoco

By default, we assume your mujoco key is at ${MUJOCO_ROOT_DIR}/bin/mjkey.txt if that is not the case you can set the environment variabe MUJOCO_KEY_PATH to the path where your mjkey.txt is.