Make `get_experiment_list_from_gene` case insensitive

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

Start in src/atldld/utils.py at the get_experiment_list_from_gene function referenced near line 205. Compare its current matching behavior with the case-insensitive web interface; done means gene searches return the same results regardless of letter case.

Written by the indexing model from the issue text.