Add code for monkeybars with randomized location of bars

- Defined a linear function to compute the right force at the time of
  jump needed to jump towards the 2nd bar.
- Bend the abdomen of the character when its coming down at the right
  side of the bar and its COM's x-coordinate is close to bar's COM's
  x-coordinate. Do the bending for a fixed amount of time.
- To make player reach the other side, release the bar when the player
  is coming down at left side of the bar and its COM's x-coordinate is
  close the bar's COM's x-coordinate