Assemble several elements at a time

Now assemble! takes a vector of elements as input. This makes it
possible to preallocate memory for common matrices making code super
fast.

test/test_common_failures.jl

@@ -7,8 +7,9 @@ using JuliaFEM.Testing
 @testset "geometry missing" begin
     el = Element(Quad4, [1, 2, 3, 4])
     pr = Problem(Elasticity, "problem", 2)
+    add_elements!(pr, [el])
     # this throws KeyError: geometry not found.
     # it's descriptive enough to give hint to user
     # what went wrong
-    @test_throws KeyError assemble!(pr, el)
+    @test_throws KeyError assemble!(pr)
 end

test/test_elasticity_2d_plane_stress_stiffness_matrix.jl

@@ -24,8 +24,9 @@ using JuliaFEM.Testing
     update!(element, "displacement load", DCTI([4.0, 8.0]))
 
     problem = Problem(Elasticity, "[0x1] x [0x1] block", 2)
-    problem.properties.formulation = :plane_stress
-    assemble!(problem, element)
+    update!(problem.properties, "formulation" => "plane_stress")
+    add_elements!(problem, [element])
+    assemble!(problem)
     K = full(problem.assembly.K)
     f = vec(full(problem.assembly.f))
 

test/test_elasticity_tet10_stiffness_matrix.jl

@@ -2,7 +2,6 @@
 # License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
 
 using JuliaFEM
-using JuliaFEM.Preprocess
 using JuliaFEM.Testing
 
 @testset "test tet10 stiffness matrix" begin
@@ -29,8 +28,9 @@ using JuliaFEM.Testing
     update!(el, "geometry", X)
     update!(el, "displacement", u)
     pr = Problem(Elasticity, "tet10", 3)
-    ass = Assembly()
-    assemble!(ass, pr, el, 0.0)
+    add_elements!(pr, [el])
+    assemble!(pr)
+    ass = pr.assembly
     Kt = full(ass.K)
     eigs = real(eigvals(Kt))
     eigs_expected = [8809.45, 4936.01, 2880.56, 2491.66, 2004.85,

test/test_elasticity_tet4_stiffness_matrix.jl

@@ -2,7 +2,6 @@
 # License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
 
 using JuliaFEM
-using JuliaFEM.Preprocess
 using JuliaFEM.Testing
 
 @testset "test tet4 stiffness matrix" begin
@@ -17,8 +16,9 @@ using JuliaFEM.Testing
     el["geometry"] = Vector{Float64}[x1, x2, x3, x4]
     u = Vector{Float64}[u1, u2, u3, u4]
     pr = Problem(Elasticity, "tet4", 3)
-    as = Assembly()
-    assemble!(as, pr, el, 0.0)
+    add_elements!(pr, [el])
+    assemble!(pr)
+    as = pr.assembly
     Kt = full(as.K)
     Kt_expected = [
          149.0   108.0   24.0   -1.0    6.0   12.0  -54.0   -48.0    0.0  -94.0   -66.0  -36.0