add condition if T=nothing

[aelligp]

This PR adds the missing condition to define T = nothing in the add_volcano function. This can be useful if one wants to add e.g. a few layers in a volcano.

  if !isnothing(T)
       Temp[ind_flat] = compute_thermal_structure(Temp[ind_flat], Grid.x.val[ind], Grid.y.val[ind], depth[ind], Phases[ind_flat], T)
  end

[github-actions]

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