added texture + volume visual

[jeromeetienne]

This pull request adds the volume visual

• a volume is processed as a bunch of points
• sometime some moire patterns occurs. As a workaround, a random jitter has been added on options.
• matplotlib is quite slow, so we do a massive downsample compared to the original data
  • the mouse brain is coming from datoviz examples https://datoviz.org/gallery/visuals/volume/

Possible conflicts

• As Importing texture + image visual #34, this PR include the Texture in gsp.core and gsp.matplotlib. This will likely create conflict in one PR when other is merged.
  • i did this to keep each PR runnable
  • i will fix the conflicts when time comes