aligator v0.8.0

This is a majorly API-breaking release following the corresponding proxsuite-nlp release, which started the work on introducing value semantics for all models in the library (without requiring clone() methods everywhere)

What's Changed

• Switch shared_ptr<T> for polymorphic<T> for several classes by @edantec in #159
• [modelling/costs] Add getResidual() templated getter for composite cost functions by @ManifoldFR in #198
• Fixing include of LU from Eigen by @quentinll in #200
• [modelling] Change CostStack backend to an unordered_map type by @ManifoldFR in #199

Full Changelog: v0.7.0...v0.8.0