Move archimedean generators to their own structs.

[lrnv]

The idea is to have

abstract type Generator end
max_monotony(Generator) = ...
\phi(Generator) = ....
\phi_inv(Generator) = ... # with an adequate default method
williamson_dist(Generator,d) = ... # with an adequate default method. 

then

struct WilliamsonGenerator <: Generator
    X::TX # the random variable 
    d::Int # the dimension in which the transformation has been done. 
end
struct ClaytonGenerator <: Generator
    theta::T
end

with the appropriate methods overloads.

And then we can construct copulas as

# for archimedeans: 
ClaytonCopula{d,T} = ArchimedeanCopula{d,ClatyonGenerator{T}}
# or even 
struct LiouvilleCopula{d,V,G}
    alpha::V # vector of dirichlet parameters
    phi::G # archimedean generator
end

it is still unclear how the other methods (fitting, tau, invtau, rho, invrho) will be implemented for specific archimedeans..

It will also easily allow for nested archimedeans, by defining e.g.

can_nest(inner::Generator, outer::Generator) = ...
struct NestedLiouvilleCopula
    phi::Generator
    content::Vector{Union{NestedLiouvilleCopula,LiouvilleCopula,Distributions.Uniform}}
end

that recursively checks possibility by using the nesting conditions through e.g.

can_nest(inner::ClaytonGenerator, outer::ClaytonGenerator) = inner.theta < outer.theta
can_nest(inner::ClaytonGenerator,outer::GumbelGenerator) = # more complicated but doable

maybe even a generic can_nest could be implemented ? I dunno if it is even possible.

So maybe ArchimedeanCopula<:LiouvilleCopula should be true ?