Initialization of optional values depending on other system models

[svchb]

As in #539 and other PRs it is a problem to initialise other systems based on the model selection:

I suggest adding in semidiscretization:

for_each(semi) do system_A 
   for_each(semi) do system_B 
      # pseudocode
      if system_A == system_B continue end
      initialize!(system_A, system_B) 
   end
end

And handling all initialization that is based on other systems in the dispatch of initialize!(system_A, system_B).

Examples of this are:

• Initialization concerning surface normals and surface tension
• Corrections