Define sparsemaps and use cached sparseindex dataset
Notes:
- Works with current siibra-python version (0.4a48 - e40973b26e2cc62035ddaff542eb86fbda8674fa)
- When the relevant branch of siibra-python is merged, it will allow clear definition of sparsemaps.
- When the ebrains dataset is created, the URLs will be updated. (PS: Gitlab connection does not require a URL)