Add a `valid_nlp` in the required methods

All the `solve!` function starts with a preamble such as
```
if !(nlp.meta.minimize)
error("This solver only works for minimization")
end
if !unconstrained(nlp)
error("Problem is not unconstrained")
end
```

maybe this should be stored in a function that is part of the JSO-compliant rules ?
For instance, `check_valid_nlp` of `check_valid_problem`.

Start by comparing the repeated solve! preambles with the JSO-compliant check_valid_nlp/check_valid_problem rules mentioned in the issue. Done means the validation is represented by the agreed shared method and the affected solve! entry points use it consistently.

Scritto dal modello di indicizzazione a partire dal testo della issue.