Add `DOI` global attribute in archive CDF output

[BaptisteCecconi]

Proposal to add an optional Global Attribute DOI. This is the DOI of the dataset the file belongs to.

There is a recommended ISTP Global Attribute for this, see here.

[CorentinLouis]

This needs to be done in src/build_serpe_skt.pro

printf,lun,"  ""DOI""          1:    CDF_CHAR     { ""https://doi.org/10.25935/"+strlowcase(doi)+"""} ."
printf,lun,""

doi will have to be pass through BUILD_SERPE_SKT function

[CorentinLouis]

This has been added to the code.
Still a need of modifying all files already produced.

Note : it is also necessary to change the Licence to CC-BY in the already produced files (see "Attribue Rules_of_Road"). This has been fixed in the develop branch for future runs.