Fix for issue #629 - Arithmetic Operations of DateTime in SPARQL

[Akash-Sharma-1]

Added the functionality for performing arithmetic operations of DateTime/Date/Time in SPARQL Queries.

This will fix the issue of #629 which requires handling of the arithmetic operation of dateTime variables in the SPARQL query. This fix also extends this operation handling to other similar Date, Time, Duration based variables. These operations are also discussed in official w3c Sparql work - (https://github.com/w3c/sparql-12/blob/master/SEP/SEP-0002/sep-0002.md)

This is the list of handled operations :

- xsd:dateTime - xsd:dateTime
- xsd:date - xsd:date
- xsd:time - xsd:time
- xsd:dateTime - (xsd:dayTimeDuration or xsd:yearMonthDuration or xsd:duration)
- xsd:dateTime + (xsd:dayTimeDuration or xsd:yearMonthDuration or xsd:duration)
- xsd:date - (xsd:dayTimeDuration or xsd:yearMonthDuration or xsd:duration)
- xsd:date + (xsd:dayTimeDuration or xsd:yearMonthDuration or xsd:duration)
- xsd:time - (xsd:dayTimeDuration or xsd:duration)
- xsd:time + (xsd:dayTimeDuration or xsd:duration)

In this PR, I have added the code to handle these operations in SPARQL queries. I have also added tests for the same operations. (Some test cases were also taken from (https://github.com/kasei/sparql-12/tree/xsd_datetime_duration/tests/xsd_functions)).

Closes #629

[coveralls]

Coverage increased (+0.04%) to 75.804% when pulling f5b7b56 on Akash-Sharma-1:master into 037ea51 on RDFLib:master.

[nicholascar]

@Akash-Sharma-1 please merge in master branch since a whole host of changes were recently merged in to remove Python 2 code. You already have a conflict here that needs to be resolved too. The merger or conflict resolution will trigger a re-checking of the code by Travis. Then it may be ready for merging.

[ashleysommer]

@Akash-Sharma-1
I resolved the merge conflict for you in your fork

[Akash-Sharma-1]

Hi @ashleysommer !
Thanks for the help !
I see in the details of Travis build for this PR and it shows all the checks as completed and passed. However, here it shows the checks as pending - this seems strange. Is that a problem with the Travis?

[ashleysommer]

@Akash-Sharma-1
You're right. That is odd. Tests have all passed, but Travis still says Pending.
Once the other maintainers have approved this, we will merge it despite what Travis says.

[Akash-Sharma-1]

@ashleysommer

Ohh great! Thank you!
Yes, I was searching and I found that its a common and known issue of Travis. Hopefully, they'll fix it.

[nicholascar]

Open Close Travis

[ashleysommer]

Sorry for delay in getting back to this. This should've been merged in after our review in July. Merging now.