Added topic to track pixel position of the center of marker

[badrobot15]

Added a topic /circle_center to track pixel coordinate of the center of the whycon marker. Useful for working with other type of markers by maintaing a single coordinate frame of reference i.e. the pixel location

[protobits]

Hi, I'm looking at your PR.
Sorry, but as it is it does not make sense: you are assuming that you have a single target (you assign the same variable inside the loop for every target). Moreover, there are unrelated changes (setting number of targets) included.
I'm not entirely against the idea of publishing image position of targets, but to accept it it would need to be correctly implemented. In other words, you can publish a pose array as with the poses but setting z = 1. Also, remove the target number setting since this is not related to this PR.

[badrobot15]

Thanks for pointing that out. I have made the required changes.
Setting the number of targets is a useful feature and hoping to see it get added in the master soon