post_process() hook for transparent CLDF schema modifications

[chrzyki]

Occasionally, the CLDF schema needs to be modified after the CLDF creation with makecldf (e.g. to address warnings about empty tables etc.). Doing so explicitly in a post_process()-hook after the CLDF creation might be more transparent, easier to track and safer.

[xrotwang]

Just looked around how to implement this. It turns out that cmd_makecldf is called from cldfbench.Dataset._cmd_makecldf, not from pylexibank. But this hook would have to be called after cmd_makecldf and before _cmd_makecldf finishes (i.e. before things are written to disk). So I think we close this as too complicated/intrusive.