Implement `NParallelTextCorpus`

[Enkidu93]

Since NParallelTextCorpus in Machine (C#) now backs the implementation for ParallelTextCorpus.GetRows() which undergirds text alignment, this is particularly important.

[johnml1135]

Is this to align SILNLP's mixed source with Serval's? It makes sense - I just want to understand the relative priority.

[Enkidu93]

Is this to align SILNLP's mixed source with Serval's? It makes sense - I just want to understand the relative priority.

I don't know where all SILNLP uses machine.py's ParallelTextCorpus and where it might have its own logic (that might be worth a closer look; probably Matthew would know), but yes, that's the general idea.