New vignette - Author ratio

[ahasverus]

Create a new vignette Author ratio to show the usage of:

• fp_compute_citation_ratio() applied on the author bibliography
• fp_retrive_author_articles() as mentioned in Anyone could compute their own ratio, by providing openalew id ? #30
• fp_retrieve_author_oa_id() to find the OA identifier of the author

This vignette should illustrate two study cases:

1. Author already has it own list of publications
2. Author needs to query OA to get this list