Allow user to manually add new journals

[ahasverus]

Feature description

Allow user to manually add new journals currently absent from fairpub. This could be achieved by adding the argument new_journals in functions computing fairness ratios.

The structure of this argument, a data.frame, must follow the structure of the internal dataset.

[nmouquet]

Yes, this could come with an adivce to the user to send the intel to the Dafnee group (do they have an email ?).