Fixed Taylor Green decay coefficient and added an example

[scaomath]

The scale for the Taylor Green example should be $\exp(-(k_x^2 +k_y^2) \nu t )$, otherwise the factor coming out from $\partial_t \mathbf{u}$ cannot be canceled with $\nu\Delta \mathbf{u}$. An example notebook computing the error with analytical solution is added as well.

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[scaomath]

@shoyer