MoleculePairPotential has problem with identical atoms

[tjjarvinen]

When creating MoleculPairPotential information of identical atoms is lost.

using PotentialFitting
using PotentialCalculation

m1 = MoleculeIdenticalInformation{AtomOnlySymbol}(["A","A","B","C","D"])
m2 =  MoleculeIdenticalInformation{AtomOnlySymbol}(["E"])
push!(m1.identical,(1,2))

mpp = MoleculePairPotential(m1,m2, LJ)

Produces

julia> mpp.mol1.identical
Identical(Set[Set([1]), Set([2]), Set([3]), Set([4]), Set([5])])

julia> m1.identical
Identical(Set[Set([2, 1]), Set([3]), Set([4]), Set([5])])

Issue seems to stem from MoleculePairPotential.mol1 having defined type. When there is no type definition everythin works as intended.

A hack around this was implemented in 7f259e7 but a better way needs to be done.

ps. This might be a issue in Julia itself and would be good idea to explore this more.