make it work w/out thermal structure specified

Author: boriskaus

src/Setup_geometry.jl

@@ -1555,7 +1555,7 @@ julia> Cart  = CartData(XYZGrid(x, y, z));
 julia> Phase = ones(Int64,size(Cart));
 julia> Temp  = fill(1350.0,size(Cart));
 # Define the trench:
-julia> trench= Trench(n_seg_xy=1, A = [400.0,400.0],B = [800.0,800.0],θ_max = 45.0, direction = 1.0, n_seg = 50, L0 = 600.0, D0 = 80.0, Lb = 500.0,d_decoupling = 100.0, type_bending =:Ribe)
+julia> trench= Trench(Start = (400.0,400.0), End = (800.0,800.0), θ_max = 45.0, direction = 1.0, n_seg = 50, L0 = 600.0, D0 = 80.0, Lb = 500.0, d_decoupling = 100.0, type_bending =:Ribe)
 julia> phase = LithosphericPhases(Layers=[5 7 88], Phases = [2 3 4], Tlab=nothing)
 julia> TsHC  = HalfspaceCoolingTemp(Tsurface=20.0, Tmantle=1350, Age=30, Adiabat=0.4)
 julia> addSlab!(Phase, Temp, Cart, trench, phase = phase, T = TsHC)
@@ -1564,7 +1564,7 @@ julia> addSlab!(Phase, Temp, Cart, trench, phase = phase, T = TsHC)
 """
 function addSlab!(Phase, Temp, Grid::AbstractGeneralGrid,  trench::Trench;        # required input
         phase::AbstractPhaseNumber = ConstantPhase(1),                       # Sets the phase number(s) in the slab
-        T::AbstractThermalStructure  = nothing )                             # Sets the thermal structure (various functions are available),
+        T::Union{AbstractThermalStructure,Nothing}  = nothing )                             # Sets the thermal structure (various functions are available),
 
     # Retrieve 3D data arrays for the grid
     X,Y,Z = coordinate_grids(Grid)