Dealing with constants in `freeze`

Right now, I think we bake constants into the code no matter what, at considerable expense. (For example, we might recompile every time the time step changes.) Maybe defaulting the other way (replace every numerical constant with a variable) is better?

cc @kaushikcfd @matthiasdiener

首先追蹤 freeze 的實作，以及程式碼產生期間如何處理數值常數。將目前的常數固化行為與以變數取代常數進行比較，包括時間步長變更時的重新編譯成本；完成條件是決定一項策略並進行相應驗證。

由索引模型根據 Issue 內容生成。