substances: rework Mixture

We need a way to represent multimer proteins and molecules consisting of subunits (proteins with RNA) and other such cases. We suggest representing these as Mixtures. Each subunit would be a MixtureComponent.

Mixture should not have its components as ManyToMany field but OneToMany relation represented by a field on each MixtureComponent that references the Mixture.

Additionally, the MixtureComponent should have concentration and its unit. For representing multimers unitless would be used (UnitOne) and instead the concentration will represent how many of these subunits the multimer has.

There should also be a way of specifying the kind of mixture. Whether it is multimer, solution, etc. For easy filtering. @markdoerr @xvelec07