IsingModel: generalize to arbitrary lattices

[carstenbauer]

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Although <:AbstractLattice is allowed as a type parameter it doesn't currently work for arbitrary lattices.

Among others, rand is assuming a cubic lattice.

julia> m = IsingModel(L=3, dims=2, l=l, neighs=zeros(Int, 0,0))
2D-Ising model, L=3 (36 sites)

julia> mc = MC(m, T=0.1234)
Monte Carlo simulation
Model: 2D-Ising model, L=3 (36 sites)
Beta: 8.1 (T ≈ 0.123)