Improve high-order horizontal derivatives #3420
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Purpose
This PR fixes a potential issue with high-order horizontal derivatives at element boundaries that was identified while writing the dycore paper. Specifically, it replaces every "strong derivative" (an ill-defined notion for our hybrid CG-DG discretization) with a "weak derivative" (defined in terms of the weak formulation and GLL quadrature rule) and a call to DSS. Hopefully this will reduce numerical oscillations at element boundaries from hyperdiffusion and the viscous sponge layer.
To-do
Find a specific example where this leads to a behavioral change.