Check if tensorboard is already running when launching

[pedroaugustosmribeiro]

After closing the Tensorboard tab, one needs to manually kill the tensorboard server processes before re-launching.
Please allow the extension to check if an instance is already running and reopen it on a tab if positive.
Also, in case of multi-user systems, if another user is already running tensorboard at default port 6000, the extension fails instead of creating a new instance at another free port, like 6001, would be nice if it tried to do so.