More or less near-future TODO/wish list

[JustRelax](https://github.com/PTsolvers/JustRelax.jl)
- [x] 2D viscous stokes
- [x] 2D visco-elsatic stokes
- [ ] 2D non-Newtonian rheology
- [ ] 2D visco-elasto-plasticity
- [ ] 2D 2-phase flow (Sidney)
- [ ] 3D Viscous stokes
- [ ] Refactor thermal diffusion (2D and 3D)
- [ ] Paraview interface for 3D viz with [WriteVTK.jl](https://github.com/jipolanco/WriteVTK.jl)
- [ ] Particles-in-cell
- [ ] Scalability tests
- [ ] Support for [GeoParams.jl](https://github.com/JuliaGeodynamics/GeoParams.jl)

[StencilInterpolations](https://github.com/albert-de-montserrat/StencilInterpolation)
- [x] CPU and CUDA (naive) bi-linear interpolation (i.e. scattering from nodes to particles)
- [ ] CPU and CUDA tri-linear interpolation (i.e. scattering from nodes to particles)
- [x] CPU and CUDA (naive) inverse (2D) weighted distance interpolation (i.e. gathering from nodes to particles)
- [ ] CPU and CUDA inverse (3D) weighted distance interpolation (i.e. gathering from nodes to particles)
- [ ] MPI support
- [ ] Multi-gpu support
- [ ] Scalability tests
- [ ] Interface for [MACadvect.jl](https://github.com/JuliaGeodynamics/MACadvect.jl/tree/main/src)

[MACadvect.jl](https://github.com/JuliaGeodynamics/MACadvect.jl/tree/main/src)
- [ ] Rename to _PiCadvect.jl_ (_markers-and-cells_ is not a thing)
- [ ] 2D RK2 advection scheme (do we actually want/need RK4 for production code?)
- [ ] 3D RK2 advection scheme