The viscosity panel was confusing for users because viscosity
is defined by the base and exponent properties in the Diffusion panel.
What this setting actually does is use a special solver algorithm that is designed for high viscosity liquids.
See 635694c0ff for details.
The below changes are designed to better represent this to the user.
- Move the "Viscosity" Panel to a sub panel of "Diffusion"
- Rename "Viscosity" to "High Viscosity Solver" to better represent what this property does
- Update `use_viscosity` tooltip to better explain that this uses a different solver algorithm.
Aaron Carlisle