Fix invalid camera parameters in gazebo_depthCamera plugin

[jchevrie]

Fix for issue #407

Fix focalLengthX and focalLengthY values returned by the plugin when asking the intrinsic parameters of the camera through rpc via the command visr get intp.
The new returned values are now pixel/meter ratios instead of meters, to be similar to the interface of the hardware cameras.

[traversaro]

Thanks @jchevrie . I think we need to first sort out what is the actually intended unit of measurements, as discussed in #407 (comment) .

[traversaro]

Given the fixes in robotology/yarp#1960, how this PR should be updated (if it needs to be updated at all)?

[jchevrie]

In the end we chose pixels as unit instead of meters, so this fix is still relevant.

I updated the code by adding the new mandatory parameter physFocalLength that was introduced.
However I set the returned value to 0.0 (meaning invalid) because I don't think we set it in gazebo for now (it has a default value 1.0 which should probably not be used as is).

[traversaro]

Great, thanks a lot @jchevrie . If @Nicogene or @barbalberto could review this PR given their expertise on this interface it would be great, thanks!

[barbalberto]

LGTM

[barbalberto]

Merged, thanks @jchevrie for your work on this topic