Added DateTime arithmetic operations for Literals - Extension of #629

[Akash-Sharma-1]

This PR adds some additional set of operations - dateTime / date / time subtraction to the Literals of RDFLib. This is an extension to the #629 issue of adding these operations to Literals in addition to the SPARQL queries.

[coveralls]

Coverage increased (+0.008%) to 75.773% when pulling 936306e on Akash-Sharma-1:dateTime-Sub-operations into 037ea51 on RDFLib:master.

[JervenBolleman]

@Akash-Sharma-1 thank you! it looks correct to me.

[nicholascar]

Hi @Akash-Sharma-1, this looks great but, what happens for other Literals that are not dates? Your function returns None fro all non dates but since you've declared a __sub__ function in term, we (maintainers) are concerned that this might override normal Python subtraction.

Can you please add a couple of tests to test_literal to show that this function doesn't bread integer, float and decimal subtraction? If so, we will merge in.

[FlorianLudwig]

@nicholascar I would suggest to throw a TypeError if subtraction is not supported for the types in question. That's what python does for other types, e.g. subtracting two strings.

[ghost]

Rebased and committed in #1870