Add spatial covariant derivative of extrinsic curvature

[guilara]

Proposed changes

Add compute tag for the spatial covariant derivative of the extrinsic curvature.

To compute WeylMagnetic (magnetic part of the Weyl tensor) we need the Spatial Covariant derivative of the Extrinsic curvature. The compute tag for WeylMagnetic was previously requiring the (partial) derivative of the extrinsic curvature. Despite the LeviCivita symbol in the definition of the WeylMagnetic, there are still terms proportional to the 3-Christoffel symbols that do not cancel. I replace the incorrect argument tag in WeylMagneticCompute. This compute tag was not used anywhere in the code.

Note: I just noticed there was already a function to compute it from gh variables, but I think I cannot make a useful compute Tag with it (bc of the partial derivatives of the gh quantities).

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Further comments

[nilsdeppe]

LGTM, couple small suggestions. Please squash immediately :)