10x volume calculations, base surface definitions. fix 3D vertical area measurements

[pierotofy]

This PR speeds up volume calculations using the measure tool by an order of magnitude. It also adds support for more options for base surface definition:

• Triangulate (default), tassellates the input point set and interpolates linearly
• Plane, fits a 3D plane minimizing the distance of the plane to all points in the set
• Average, uses the mean Z of all points
• Highest, uses the highest Z of all points
• Lowest, uses the lowest Z of all points

This PR also removes grass-core as a dependency, which was an obstacle to a possible future port to native macOS.

It moves the changedetection and elevationmap plugins to https://github.com/OpenDroneMap/WebODM-Plugins-Archive. I don't know who is using these plugins, but they have dependencies to the grass code that is now removed. It should be possible to update the plugins by using the more up-to-date run_function_async function (instead of relying on the previously used grass context/engine), but will require some work. The plugins are in need of a maintainer.

[Saijin-Naib]

Also makes rolling this up for Alpine a bit easier for me 🤓

This is quite an exciting rework!

[pierotofy]

Fixes #1431

[pierotofy]

Fixes #1421