BUG: Fix display in Jupyter Notebook

[bnmajor]

Moving the viewer initialization into the run function prevents the viewer from being displayed as a dialog in the Jupyter Notebook and instead displays it inline as expected. This also prevents the multiple window creation that was happening in Colab. From the ImJoy docs:

• setup() function: executed when a plugin is loaded and it initializes for the first time.
• run() function: will be called each time a plugin is executed. When executed, an object (for JavaScript) or a dictionary (for Python) with context (named ctx) will be passed into the function. The returned result will be displayed as a new window or passed to the next op in a workflow.

I am not entirely sure why this change would affect the way that the output is displayed but this change better matches the examples that ImJoy provides (computation plugin, itk-vtk-viewer, image viewer)