Add flag to generate FST file from NVC, without spawning GTKWave

[kmtaylor]

I find it convenient to be able to generate a FST file when using NVC, without spawning GTKWave (-g). Added a command line argument to achieve this.

[oscargus]

I'm working on (edit: planned to work on) handling this in a unified way for both NVC and GHDL. (ghdlgenerates a wave file if--gtkwave-fmt` is provided, although it isn't documented as far as I can tell.)

I suggest that you either implement this using the --gtkwave-fmt (which will be renamed to --viewer-fmt in #1002) or add a flag, e.g., --waves that generates waveform files for both GHDL and NVC, see #1003 for a bit of discussion of another use case.

However, @LarsAsplund may want a say here what the way forward is.

[kmtaylor]

Ok - so for now I'll wait to see what happens with #1002 and #1003.