Add data-free methods for add_vertex! and add_edge!

[thchr]

Following discussion with @gdalle on Slack, here's a suggestion to have data-free versions of add_vertex! and add_edge! if the corresponding data-types are Nothing.

@gdalle mentioned that this could potentially be for any union type, but I wasn't completely clear on the implications of that. This was intended as the case where e.g., VertexData = Union{T, Nothing} where T? Should we add that?

[gdalle]

@gdalle mentioned that this could potentially be for any union type, but I wasn't completely clear on the implications of that. This was intended as the case where e.g., VertexData = Union{T, Nothing} where T? Should we add that?

On second thought this might be confusing, since T can also have a default value the user wants to use. Let's stick to pure Nothing for now

[gdalle]

Could you maybe add some small tests?

[thchr]

I added some tests now. I didn't add them to the tutorials, partly because I'm not familiar with the tooling there, partly because this seems not sufficiently important to include in a tutorial.

[gdalle]

I added some tests now. I didn't add them to the tutorials, partly because I'm not familiar with the tooling there, partly because this seems not sufficiently important to include in a tutorial.

I agree :) Could you maybe just put these tests in a separate file, like test/misc.jl, which you include from test/runtests.jl?

[thchr]

I agree :) Could you maybe just put these tests in a separate file, like test/misc.jl, which you include from test/runtests.jl?

Sure: done.

[gdalle]

I fixed the constructors because your tests used the old version, don't forget to run the tests locally before asking CI to do the same 😉 we should be good to merge once they pass

[thchr]

I've been shamelessly writing this entirely on Github's editor - so nothing checked locally :D. Anyway, more formatting issues finally made me download it locally to run JuliaFormatter; tests now also pass locally.

[codecov]

Codecov Report

Merging #48 (4ab0b2c) into master (f546d96) will increase coverage by 0.10%.
The diff coverage is 100.00%.

❗ Current head 4ab0b2c differs from pull request most recent head a10deba. Consider uploading reports for the commit a10deba to get more accurate results

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
+ Coverage   92.72%   92.83%   +0.10%     
==========================================
  Files           7        7              
  Lines         275      279       +4     
==========================================
+ Hits          255      259       +4     
  Misses         20       20              

Impacted Files	Coverage Δ
src/graphs.jl	87.61% <100.00%> (+0.49%)	⬆️

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