Add Bernoulli random graph

[aurorarossi]

With this PR I add the Bernoulli random graph and the $\rho$-correlated Bernoulli random graphs generators.

[codecov[bot]]

Codecov Report

Merging #200 (64fc9ec) into master (34743a5) will decrease coverage by 0.05%. The diff coverage is 100.00%.

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Additional details and impacted files

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- Coverage   97.28%   97.24%   -0.05%     
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  Files         115      114       -1     
  Lines        6789     6610     -179     
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- Hits         6605     6428     -177     
+ Misses        184      182       -2     

[simonschoelly]

Great! have you also thought about some functions for the directed case?

[aurorarossi]

Great! have you also thought about some functions for the directed case?

I have just implemented the generator described in this paper , I have no knowledge of a directed case.

[gdalle]

@aurorarossi is this good to go?

[gdalle]

Side note, this might be linked to #212