Add obstacles tutorial

[blegat]

Taken from Joey Huchette at SIAM Opt 2017

@joehuchette do you still have the code?

[codecov]

Codecov Report

Merging #257 (47fbe4b) into master (a4e1a6a) will increase coverage by 0.29%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #257      +/-   ##
==========================================
+ Coverage   78.17%   78.46%   +0.29%     
==========================================
  Files          36       36              
  Lines        1700     1788      +88     
==========================================
+ Hits         1329     1403      +74     
- Misses        371      385      +14     

Impacted Files	Coverage Δ
src/variable.jl	82.85% <0.00%> (-5.03%)	⬇️
src/Certificate/Symmetry/utils.jl	61.29% <0.00%> (-3.00%)	⬇️
src/Certificate/Sparsity/sign.jl	100.00% <0.00%> (ø)
src/Certificate/Sparsity/monomial.jl	100.00% <0.00%> (ø)
src/Certificate/Sparsity/variable.jl	100.00% <0.00%> (ø)
src/Certificate/Symmetry/block_diag.jl	0.00% <0.00%> (ø)
src/Certificate/Symmetry/wedderburn.jl	0.00% <0.00%> (ø)
src/Bridges/Constraint/sos_polynomial.jl	100.00% <0.00%> (ø)
...c/Bridges/Constraint/scaled_diagonally_dominant.jl	0.00% <0.00%> (ø)
src/Certificate/newton_polytope.jl	99.19% <0.00%> (+0.05%)	⬆️
... and 8 more

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