return a sparse form of the hessian using scipy.sparse
this function returns the hessian in compressed sparse column (CSC) form
- Advantages of the CSC format:
- efficient arithmetic operations CSC + CSC, CSC * CSC, etc.
- efficient column slicing
- fast matrix vector products (CSR, BSR may be faster)
- Disadvantages of the CSC format:
- slow row slicing operations (consider CSR)
- changes to the sparsity structure are expensive (consider LIL or DOK)