Sound speed limiter on TDC?

@orlox suggested that we impose a (smooth) limit on the convection speed in TDC, and implemented this in the `tdc_gamma_no_cs` branch.

I'd just like to have a bit of discussion about the physics involved here before we merge that in. In particular, I'd like to hear from @rsmolec on whether or not there is actually physics limiting convection to the sound speed, and to how consistent any additional physics we might include to create such a limit is with the physics already in TDC.