Adding time-varying wind effects

[pmanish1234]

In the windy.sdf file, we can change linear velocity of the wind using

<wind>
      <linear_velocity>5 2 0</linear_velocity>
</wind>

What attributes can be used for simulating time-varying wind effects such as sinusoidal wind?

[beniaminopozzan]

Give a look at #130 :)