Add combining rules for intermolecular interaction parameters about lumol

[Luthaf]

I agree this is nice to have, but I would not add a separated table for that. As combining rules mainly (only?) make sense for Lennard-Jones, we can reuse the lj key:

[input]
version = 1

[[pairs]]
atoms = ["A", "A"]
lj = {sigma = "... ", epsilon = ..."}

[[pairs]]
atoms = ["B", "B"]
lj = {sigma = "... ", epsilon = ..."}

[[pairs]]
atoms = ["A", "B"]
# Use previously defined parameters here
lj = {combine = "LorentzBerthelot"}

That plus #7 allow to use combining rules for all unspecified parameters:

[[pairs]]
atoms = ["*", "*"]
lj = {combine = "LorentzBerthelot"}