Add linear.y to velocity_smoother

[nlimpert]

Hi,

I really appreciate your work on the velocity_smoother - it rocks!
For a holonomic platform I'd like to make use of it too.
Although it seems to lack velocity smoothing in the linear.y dimension.

Do you think it's possible to implement it?

Best regards

Nick

[stonier]

Shouldn't be hard, feel free to make a pull request.