Add logging functionality

[anriseth]

I'd like to see a flexible logging / tracing functionality here.

Currently there is little info provided when things go wrong inside the linesearches. Finiteness tests such as #101 should (optionally) warn the user that something is wrong.

[pkofod]

Yes, but let’s see if we can’t make something a bit more elegant than optim

[anriseth]

let’s see if we can’t

I hope we can :)

Edit: I now realise that the redundant negative was probably intended 😆

[timholy]

See #177