Add `return nothing` statements to examples before runic formatting

[j-fu]

Automatic runic formatting would turn code like

function flux(y,u,edge,data)
  y[1] = u[1,1] - u[1,2]
  y[2] = u[2,1] - u[2,2]
end

into

function flux(y,u,edge,data)
  y[1] = u[1,1] - u[1,2]
  return y[2] = u[2,1] - u[2,2]
end

This code is less clear for new users as it should be, as they may think that the returned
value is somehow needed.

Essentialy, this will affect all the examples which also serve as user docs.

Beore runic autoformatting, this kind of code should be turned (manually) into

function flux(y,u,edge,data)
  y[1] = u[1,1] - u[1,2]
  y[2] = u[2,1] - u[2,2]
  return nothing
end

Possibly, a good editor could help a bit...