feat: Support fit_power_law(implementation = "plfit.p") to compute the P-value

[krlmlr]

Closes #1158.

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[maelle]

I see I was tasked with refining the interface as needed, but I'd need some more pointers/context.

[Antonov548]

@krlmlr I have added R_igraph_power_law_fit_new() and seems R_igraph_power_law_fit() not used anymore.

I have created R_igraph_power_law_fit_new() which doesn't use R_igraph_power_law_fit() to create result list with correct amount of fields.

I assume there is no tests for this function right now? I think we might need to add it using snapshot?

[krlmlr]

Thanks, looks good!

[krlmlr]

We could also call the generated function and use lengthgets() to add an element to the returned list. This is better because we then would still rely on autogeneration for the better part of the code here.

[Antonov548]

we can't use generated function. To compute p-value we need return value from igraph_power_law_fit C function, but we lose this data if use generated function. I think using generated function give to much overhead.

[maelle]

Thanks @Antonov548!

@krlmlr do you have further thoughts on this?