Add back tributary area analysis

[ioannis-vm]

Relying on scikit-geometry for tributary area analysis is problematic, since it can't be directly installed using pip.

• Dropping support for tributary area analysis. Users will now have to specify slab mass and weight manually to elements and nodes.
• Will consider adding it back at some point with a custom implementation of the straight skeleton generation algorithm, and potentially with more control on where the slabs are located, as well as their geometry and support fixity.