[GemPy] widget enhancements

[banesullivan]

These are improvements from @Leguark's changes in #410 to make the widgets a bit more robust and ready for advanced use in downstream dependencies (namely GemPy). This also greatly improves the testing of the widgets

@Leguark: you can now just pass implicit=False to the add_plane_widget() method and it will handle setting up the simple plane widget with callbacks and all - I'm going to keep the implicit plane widget as the default as it is a bit more user-friendly. Also, this adds a flag, pass_widget to all widget methods that allows you to pass the widget to the callback for advanced usage

[Leguark]

Thank you for update this. I had to make a few changes to make it run properly but I do not have the rights to push to this branch directly. Do I make a pull request to this branch or how do you prefer.

Also another problem I found a small bug with add_bounds_axes. I guess it is not worth it a different pull request for that. I am not sure how strict you are with these things

[banesullivan]

I just sent you an invite to join the org so that you can push to this branch! Feel free to make changes here

As for the bug in the add_bounds_axes, if it’s small, you can add it here - maybe do it as its own commit so I can see what it is

_{Sent with GitHawk}

[banesullivan]

Looks good to me. @Leguark, I'm going to merge