Add volume data plugin and adapt to current core API

[alxvth]

• Add volume data plugin from: https://github.com/Rsnelllenberg/core_volume
• Streamline cmake setup, e.g., automatically fetch hnswlib if not provided
• Refer to the original project in the README, in addition to this repo being a fork
• Adapt the plugins to the current core (ManiVault 1.4 API):
  • Adjust scatterplot bounding boxes (new core API quirk) in DVRTransferFunction view, see the respective Scatterplot PR
  • Use previous quad vertex shader (from before some breaking changes in the core) to fix runtime issues

TODO:

• Fix CI build failure, do not build on MacOS
• Add PluginInfo.json Let's do this in a separate PR

This seems to work as intended now: