Implement basis factory

[rafaelrojasmiliani]

• [x] remove gsplines::basis::string_to_basis
• [x] Implement

  std::shared_ptr<Basis> get_basis(const std::string& _name, std::size_t _dim, const std::vector<double> &_params){
  if(_name == 'lagrange'){
  return BasisLagrange::get(_params);
  }else
  if(_name== 'legendre'){
   return BasisLegendre::get(_dim);
  }
  }

• [x] Implement BasisLegendre::get(std::size_t _dim)
• [x] Implement BasisLagrange::get(const std::vector<double> &_points)
• [x] Implement BasisLagrange::get(const Eigen::VectorXd &_points)

[rafaelrojasmiliani]

Related to #40