[FEAT] Have load_data create a new viewer if one does not exist

[eteq]

Jdaviz component

Imviz

What is the problem this feature will solve?

When following a workflow that involves comparing images side-by-side, it's often convenient to make viewers at the same time as loading the data. It's just syntactic sugar on some level but it seems unambiguous and requires knowing less of the API to get started.

Describe the desired outcome

When calling load_data(filename, data_label=label, show_in_viewer=label), if label is not already a viewer, automatically call create_image_viewer.

Additional context

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