Add sign symmetric tutorial (#258)

* Add sign symmetric tutorial

* Exclude tests

Author: blegat

docs/src/tutorials/Sparsity/sign_symmetry.jl

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+# # Term sparsity
+
+#md # [![](https://mybinder.org/badge_logo.svg)](@__BINDER_ROOT_URL__/generated/Sparsity/sign_symmetry.ipynb)
+#md # [![](https://img.shields.io/badge/show-nbviewer-579ACA.svg)](@__NBVIEWER_ROOT_URL__/generated/Sparsity/sign_symmetry.ipynb)
+# **Adapted from**: Example 4 of [L09]
+#
+# [L09] Lofberg, Johan.
+# *Pre-and post-processing sum-of-squares programs in practice*.
+# IEEE transactions on automatic control 54, no. 5 (2009): 1007-1011.
+
+using Test #src
+using DynamicPolynomials
+@polyvar x[1:3]
+
+# We would like to determine whether the following polynomial is a sum-of-squares.
+
+poly = 1 + x[1]^4 + x[1] * x[2] + x[2]^4 + x[3]^2
+
+# In order to do this, we can solve the following Sum-of-Squares program.
+
+import CSDP
+solver = CSDP.Optimizer
+using SumOfSquares
+function sos_check(sparsity)
+    model = Model(solver)
+    con_ref = @constraint(model, poly in SOSCone(), sparsity = sparsity)
+    optimize!(model)
+    @test termination_status(model) == MOI.OPTIMAL #src
+    println(solution_summary(model))
+    return gram_matrix(con_ref)
+end
+
+g = sos_check(Sparsity.NoPattern())
+@test g.basis.monomials == [x[1]^2, x[1] * x[2], x[2]^2, x[1], x[2], x[3], 1] #src
+g.basis.monomials
+
+# As detailed in the Example 4 of [L09], we can exploit the *sign symmetry* of
+# the polynomial to decompose the large positive semidefinite matrix into smaller ones.
+
+g = sos_check(Sparsity.SignSymmetry())
+monos = [sub.basis.monomials for sub in g.sub_gram_matrices]
+@test length(monos) == 3 #src
+@test [x[1], x[2]] in monos #src
+@test [x[3]] in monos #src
+@test [x[1]^2, x[1] * x[2], x[2]^2, 1] in monos #src