Add logical_divergence function, elliptic StrongLogical DG formulation

[nilsvu]

Proposed changes

This variant of the elliptic DG formulation moves the Jacobian into the divergence like this:

$$ \partial_i F^i = \frac{1}{J} \partial_\hat{\imath} J {J^{-1}}^{\hat{\imath}}_j F^j $$

This formulation makes the strong scheme work as well as the weak scheme for extremely stretched grids (initial data with large outer boundaries).

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Further comments

[nilsdeppe]

LGTM, a couple suggestions you can squash immediately.