LLSModel hess_structure! not implemented

Originally https://github.com/JuliaSmoothOptimizers/NLPModels.jl/issues/273 by @vepiteski 

Code compiled using 0.10.0 but now fails under 0.12.2. The following reproduces the bug.

```
Z = rand (3,2)
 v = rand(3)
 model = LLSModel(Z,v)
 hess(model,zeros(2)
```