Consistency with information standard

[joncison]

The mandatory attributes should be consistent with what's eventually settled for the emerging information standard (https://github.com/bio-tools/biotoolsSchemaDocs/blob/master/information_requirement.rst), including:

• Make “Operation” etc. optional (depending on decision)
• Add “Unpublished” and “Unknown” to DOI, PMID and PMCID fields (see N/A publications biotoolsRegistry#200)

[joncison]

The constraints will be relaxed to the level of "SPARSE" in the information standard.

[joncison]

function->operation, labels->toolType and labels->topic now optional (0...many in all cases)

"Not available" added to DOI, PMID and PMCID by #82