fix: RDF serialisation, prov:atTime attribute

[cmaumet]

Hi @satra, @trungdong,

This is a small fix to the RDF serialisation (cf. #49).

I think that the prov:time attribute in a Generation should be replaced by a prov:atTime attribute (cf. https://www.w3.org/TR/prov-o/#atTime). @satra: do you agree with this?

This issue was identified when testing the prov RDF serialisation as part of https://github.com/incf-nidash/nidmresults-fsl. Apart from this small issue and RDFLib/rdflib#655, everything looks fine to me!

[trungdong]

Unfortunately, this only applies to PROV-O. Other representations still use prov:time.

You would probably want to have a PROV-O specific constant instead of changing PROV_ATTR_TIME.

[satra]

@cmaumet - i think we can take care of this in the rdf serialization. there are a few things that do this for other PROV-O concepts.

[cmaumet]

Yes, if I remember well prov:location/prov:atLocation is another one...

[satra]

@cmaumet - the atLocation was taken care of, but not atTime - i'm running a separate set of tests now to ensure that these are all covered.

[cmaumet]

I am closing this PR as @satra has now fixed the issue in #90.