Adding support for named graphs

[QuentinJanuel]

This pull request will aim to address the issue #3

[bb010g]

Is this based on HDTQ (previous implementation: https://github.com/Rummy23/hdtq-java)?

[QuentinJanuel]

@bb010g Yes

[D063520]

Hi @QuentinJanuel , I saw that you added named graph cat test. This is fine, but what we would need more is the following:

• index one hdt
• run all different types of iterators and check if the result is the right one

this is the main test we need. You can ping me if it is not clear ...

[QuentinJanuel]

Sure no problem, I will prioritize these tests then

[D063520]

thank you for the contribution and the reviews!