Equivalent of Courant/Fourier criterion for transported scalars.

Currently, the listing contains the Courant and Fourier numbers for the momentum equation, and the combined Courant/Fourier criterion. It could be interesting to also have this combined Courant/Fourier criterion for transported scalars. This might be especially relevant for buoyant scalars.

该 issue 未指定任何文件、测试或入口点。首先定位现有的 Courant 和 Fourier 列表以及组合的动量方程判据，然后追踪输运标量和浮力标量是如何表示的。完成的标准是为输运标量定义并公开一个等效的组合判据，并对相关情况进行验证。

由索引模型根据 Issue 内容生成。