Enhance driver functions for niRFSG_WriteArbWaveform

[Yarzey]

In the RF Signal Generator driver declaration, (i.e. niRFSG.h), there are several instrument driver function declarations that generates arbitrary waveforms. Namely;

   niRFSG_WriteArbWaveform   (which should be named "niRFSG_WriteArbWaveformF64" for uniformity)

   niRFSG_WriteArbWaveformF32

   niRFSG_WriteArbWaveformComplexF64

   niRFSG_WriteArbWaveformComplexF32

   niRFSG_WriteArbWaveformComplexI16

Although, some of them are only declarated but not installed yet.

I strongly propose to add to this list, a new function "niRFSG_WriteArbWaveformI16", which seems to be the most easiest and often used in many cases.

(I don't understand why it is not in the declaration in the first place.)

I kindly ask for your consideration.