Make `get_experiment_list_from_gene` case insensitive

[jankrepl]

https://github.com/BlueBrain/Atlas-Download-Tools/blob/b08093f5bebf53c7491ce1b314382eeb0e78d656/src/atldld/utils.py#L205

The motivation behind making the search case insensitive is that the web interface is also case insensitive. http://mouse.brain-map.org/

However, it is not a must