Add tool CGConvert to convert between file format versions

[pearzt]

This is pretty much just what the title says. I was looking for such a tool to generate test cases with file format version 3 to pymetacg, didn't find one, so I cobbled this together.

[jplehr]

Initial review.

The cxxopts comment is a wish-thing. I do understand that it may look like overkill for such a simple tool. Please look at it though and make a reasonable decision.

[sebastiankreutzer]

LGTM. There is always the question about tests, but in this case I think the conversion is handled by the graph lib unit tests.
IMO, cxxopts is not necessary here due to the simplicity (we also don't use it for cgmerge(2)).

[jplehr]

IMO, cxxopts is not necessary here due to the simplicity (we also don't use it for cgmerge(2)).

Yeah, I can understand that point of view. I mostly think that having a uniform way of doing things is beneficial. So, IMHO, we should strive towards a more uniform way, i.e., use cxxopts for everything instead of having this arbitrary "this is simple enough" metric.

Not a blocker, and not the most important thing. Still my point of view.

[TimHeldmann]

Very useful, thank you!
Looks good to me, just two questions.