Make EulerEq2d run with Kelvin Helmholtz test

• fixed dg weak form computation in 2d for special case when one component of the flux is zero
• fixed/implemented flux and ldg flux equations for 2d
• converted all max calls to absmax calls in equations.jl
• extracted numerical utilities from utils.jl into numutils.jl

The IntegrationTests/refs/euler2d_kelvin_helmholtz_entropyav.toml test now runs with polynomial orders n=1 (up to n=4; but not fully tested). Unfortunately, atm this runs pretty slow, so it is not yet added as ref test, but goal is to make it one in the future.

added 1d 2d bugfix equation labels

added 1 commit

• f576becf - add TODO on bdryconds not being implemented for AV 2d

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merged

mentioned in commit 440d899d

mentioned in commit 2dda124f