Refactoring HDT Cat by creating reusable components and signatures

[AlyHdr]

In this pull request we worked out a new version of HDT cat that eliminates some unnecessary code and unifies the classes used to iterate over the sections of the dictionary in order to create intersections and unions out of the two given files, in this pull request we introduced some main components:

• CatWrapper: Wraps around an iterator of a dictionary section and returns a defined object CatElement which always contains the next element to be added to the new section + the mapping which points to which sections an element may go or leave to.
• CatIntersection: Gets the intersection of two given dictionary sections.
• CatUnion: Gets the union of two given dictionary sections.
• CatElement: an element representing the next entity(String) to be added in the new section.
• SectionUtil: creates a section out of a given list of iterators of intersections and unions while filing the necessary mappings to translate the triple IDs after the creation of the new dictionary. This is used to create any section of the dictionary and can be used to any other structure.

Some extra tests has been added triggering a bug that was discovered during the implementation of this new version, which comes out when using specific string in the RDF files. The problem was in comparing CompactStrings.

[D063520]

I'm in favour of this refactoring. The code was difficult to read and it is much more logic.
NOTE: this fix should be integrated though .... #128 which maked HDTCat work also on windows. The original message in the pull requests states clearly what the changes are

[mielvds]

Hi! Thanks for the contribution, happy to merge, but like @D063520 mentions, you are reverting some of the fixes from #128, namely catching the exceptions when calling Files.delete(). Basically, leave hdt-java-core/src/main/java/org/rdfhdt/hdt/hdt/impl/HDTImpl.java untouched.

[AlyHdr]

Changes has been made to take #128 into account.