Implement a method to set the degree of contribution of authors

[arnaudgallou]

E.g. $set_contribution_degrees() which would have 2 purposes:

• Support Quarto's degree of contribution in PlumeQuarto.
• Give more control on how to order authors when using $get_contributions() for Plume. For example, authors set as lead contributors would be listed first for a given role. Replaced with Implement a method to force one or more contributors' names to appear first in the contribution list #34.
• Set contribution degrees to build contribution diagrams in Plume. See Add support for contribution diagrams #35.