return gene groups in abagen.get_expression_data

[JohannesWiesner]

Would be very nice if abagen.get_expression_data would also return a list-like object of gene-groups (e.g. dopaminergic, noradrenergic, cholinergic, etc) that would allow for a quicker subsection of the returned dataframe which has ~ 15.000 columns. One could also think about returning a multi-index dataframe where the first level denotes the gene-group and the second level denotes the genes (as it is currently).