More consistent output from link functions

[lbittarello]

No description provided.

[lbittarello]

I removed the __new__ method of TweedieLink, because it's quite odd to instantiate a different class for a certain parametrisation and the benefits don't warrant it to my mind. Let me know if I should revert it.

We could also remove the calls to _asanyarray and have the link functions preserve the input type, which is consistent in a different way.

[lbittarello]

In a similar vein, should we turn PoissonDistribution, GammaDistribution and NormalDistribution into fully fledged classes instead of inheriting from TweedieDistribution? The current setup makes it awkward for downstream packages to inherit from them: for example, if I want to modify both PoissonDistribution and TweedieDistribution, I either need to live with diamond inheritance (PoissonDistribution inherits from glum.PoissonDistribution, which inherits from glum.TweedieDistribution, and from my TweedieDistribution, which also inherits from glum.TweedieDistribution) or to following a different pattern from Glum and not have my PoissonDistribution inherit from my own TweedieDistribution.

[jtilly]

In a similar vein, should we turn PoissonDistribution, GammaDistribution and NormalDistribution into fully fledged classes instead of inheriting from TweedieDistribution?

No objections.

[lbittarello]

I'll leave it to a separate PR. :)

[lbittarello]

@jtilly, @MarcAntoineSchmidtQC, any objections to this PR?