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`.