Test if RStudio is used when the GUI is launched

The GUI is sometimes unstable with the latest versions of RStudio. The session sometimes aborts.
This happens most often when playing the time series on the "Select the target date" of the "Model diagram" panel.
This does not seem to happen when R is run from a terminal or from the Rgui.

While waiting to understand the origin of this problem, it would be better to warn users when loading the package and when launching the Gui using ShinyGR().