Visualize full brain morphologies with huge sizes

[marwan-abdellah]

We recently received a dataset for the full mouse brain that has more than 5 million samples. Using polylines to visualize such a huge dataset is not applicable without commodity GPUs.
To resolve this issue and visualze the dataset, we can sample the loaded section list and visualize a subset of the morphology.

Note that the analysis will be applied on the entire dataset, while the resampled one is only used for the visual representation.

We should also add a note in the UI that we only visualze a subset of the morphology.