Add Positivity Preserving Feature for GaussSBP nodes

Hello,

The positivity preserving feature `PositivityPreservingLimiterZhangShu` does not seem to work for `GaussSBP()` with the time stepper `SSPRK43()` or ` CarpenterKennedy2N54`

1D Compressible Euler example with `GaussSBP()`:

```
using Trixi
using OrdinaryDiffEq

gamma_gas = 1.4
equations = CompressibleEulerEquations1D(gamma_gas)

###############################################################################
# setup the GSBP DG discretization that uses the Gauss operators from
# Chan, Del Rey Fernandez, Carpenter (2019).
# [https://doi.org/10.1137/18M1209234](https://doi.org/10.1137/18M1209234)

# Shu-Osher initial condition for 1D compressible Euler equations
# Example 8, https://www.sciencedirect.com/science/article/pii/0021999189902222
function initial_condition_shu_osher(x, t, equations::CompressibleEulerEquations1D)
x0 = -4

rho_left = 27/7
v_left = 4*sqrt(35)/9
p_left = 31/3

# Replaced v_right = 0 to v_right = 0.1 to avoid positivity issues.
v_right = 0.1
p_right = 1.0

rho = ifelse(x[1] > x0, 1 + 1/5*sin(5*x[1]), rho_left)
v = ifelse(x[1] > x0, v_right, v_left)
p = ifelse(x[1] > x0, p_right, p_left)

return prim2cons(SVector(rho + 0.1*rand(), v + 0.1*rand(), p + 0.1*rand()), equations)
end

initial_condition = initial_condition_shu_osher

surface_flux = flux_lax_friedrichs
volume_flux = flux_ranocha

polydeg = 3
basis = DGMultiBasis(Line(), polydeg, approximation_type = GaussSBP())

indicator_sc = IndicatorHennemannGassner(equations, basis,
alpha_max = 0.5,
alpha_min = 0.001,
alpha_smooth = true,
variable = density_pressure)
volume_integral = VolumeIntegralShockCapturingHG(indicator_sc;
volume_flux_dg = volume_flux,
volume_flux_fv = surface_flux)

dg = DGMulti(basis,
surface_integral = SurfaceIntegralWeakForm(surface_flux),
volume_integral = volume_integral)

boundary_condition = BoundaryConditionDirichlet(initial_condition)
boundary_conditions = (; :entire_boundary => boundary_condition)

###############################################################################
# setup the 1D mesh

cells_per_dimension = (64,)
mesh = DGMultiMesh(dg, cells_per_dimension,
coordinates_min = (-5.0,), coordinates_max = (5.0,),
periodicity = false)

###############################################################################
# setup the semidiscretization and ODE problem

semi = SemidiscretizationHyperbolic(mesh, equations, initial_condition,
dg, boundary_conditions = boundary_conditions)

tspan = (0.0, 2.0)
ode = semidiscretize(semi, tspan)

###############################################################################
# setup the callbacks

# prints a summary of the simulation setup and resets the timers
summary_callback = SummaryCallback()

# analyse the solution in regular intervals and prints the results
analysis_callback = AnalysisCallback(semi, interval = 100, uEltype = real(dg))

# handles the re-calculation of the maximum Δt after each time step
stepsize_callback = StepsizeCallback(cfl = 0.1)

# collect all callbacks such that they can be passed to the ODE solver
callbacks = CallbackSet(summary_callback, analysis_callback, stepsize_callback)

# For future work, add PositivityPreservingLimiterZhangShu for DGMulti with Gauss nodes
stage_limiter! = PositivityPreservingLimiterZhangShu(thresholds=(5.0e-6, 5.0e-6),
variables=(Trixi.density, pressure))

# ###############################################################################
# # run the simulation

sol = solve(ode, SSPRK43(stage_limiter!), adaptive = true, callback = callbacks, save_everystep = true)
```

Start with the GaussSBP-based DGMulti example in the issue and trace how PositivityPreservingLimiterZhangShu is applied with SSPRK43 and CarpenterKennedy2N54. Compare the limiter's current behavior for GaussSBP nodes with the existing DGMulti setup; done means the supplied 1D Compressible Euler simulation runs without positivity failures for the requested time steppers.

Written by the indexing model from the issue text.