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)
```

Reproduce the failure with the LLSModel example in the issue, correcting the missing closing parenthesis if needed. Start by locating the LLSModel hess_structure! entry point and the hess call it supports; determine the expected behavior from nearby model APIs. Done means the example works under Julia 0.12.2 and the relevant behavior is covered by a test.

Written by the indexing model from the issue text.