Support affiliation fields like elsevier template

[cderv]

This closes #509 by correctly supporting affiliation fields as explained in the doc and template

• https://www.elsevier.com/authors/policies-and-guidelines/documents/elsdoc-1.pdf
• https://ctan.crest.fr/tex-archive/macros/latex/contrib/elsarticle/elsarticle-template-harv.tex

%% use optional labels to link authors explicitly to addresses:
%% \author[label1,label2]{}
%% \affiliation[label1]{organization={},
%%             addressline={},
%%             city={},
%%             postcode={},
%%             state={},
%%             country={}}
%%
%% \affiliation[label2]{organization={},
%%             addressline={},
%%             city={},
%%             postcode={},
%%             state={},
%%             country={}}

the skeleton has been updated to show how to pass affiliation either using the fields or using raw latex. Passing a simple string like

  - code: Another University
    address: Department, Street, City, State, Zip

was not showing the comma in the output (it would probably need to be escaped)

[cderv]

PR completed by a7f0ec01a788725018fea3e7497f724359bcc329 to fix skeleton.Rmd