Revisit `EulerGravity` with LDG, `linear_structure` & Krylov.jl

[DanielDoehring]

#2609 implemented

Trixi.jl/src/semidiscretization/semidiscretization_hyperbolic_parabolic.jl

Lines 364 to 382 in b5ecc2a

	"""
	linear_structure(semi::SemidiscretizationHyperbolicParabolic;
	t0 = zero(real(semi)))
	Wraps the right-hand side operator of the hyperbolic-parabolic semidiscretization `semi`
	at time `t0` as an affine-linear operator given by a linear operator `A`
	and a vector `b`:
	```math
	\\partial_t u(t) = A u(t) - b.
	```
	Works only for linear equations, i.e., equations with `have_constant_speed(equations) == True()`.
	This has the benefit of greatly reduced memory consumption compared to constructing
	the full system matrix explicitly, as done for instance in
	[`jacobian_fd`](@ref) and [`jacobian_ad_forward`](@ref).
	The returned linear operator `A` is a matrix-free representation which can be
	supplied to iterative solvers from, e.g., [Krylov.jl](https://github.com/JuliaSmoothOptimizers/Krylov.jl).
	"""

which, together with the LDG parabolic solver enables solving elliptic equations on TreeMeshes.

Thus, one could revisit the Euler-Gravity example and switch out the explicit iteration into steady state of the hyperbolic diffusion equations.

[ranocha]

See also #419