Limited-memory QN model when the partial separability is unproductive

[paraynaud]

When storing the partitioned matrix is costlier than storing a dense matrix, set :

• PLBFGSNLPModel to LBFGSNLPModel ;
• PLSR1NLPModel to LSR1NLPModel ;
• PLSENLPModel to either LBFGSNLPModel or LSR1NLPModel .