Release v0.6

[ranocha]

• Migrate neural network-based indicators to new repository #1701
• Add new non-conservative flux for standard and two-layer shallow water equations #1508
• Breaking changes to parabolic terms (add gradient variable type parameter to AbstractEquationsParabolic #1409 and removing inv_h from penalty::LaplaceDiffusion2D)

Left for the future, e.g., #1726

• Switch default HLL wave speed estimate to Davis, see Hll 2 wave improvements #1545
• Unify boundary condition specification #1510
• Unify signatures of mesh constructors #1194
• Discuss names of serial/parallel mesh types #1046
• wrap RHS to reduce latency with OrdinaryDiffEq.jl #1255
  • If we want this to work smoothly, we will have to break plot(sol)
• Boundary conditions on nonconservative terms not supported #1445
• Move some advanced shallow water stuff to TrixiShallowWater.jl