[ADQL] Fix merge side-effects (e.g. '' were not translated any more as a single ' ; NullPointerExceptions when building positions) + transform the test main class for positions into a JUnit test case + Fix some position mistakes.
(note: a tabulation character seems to be interpreted by JavaCC as 8 characters)

ADQLObject has now a new function: getPosition(). To allow it, the parser and all ADQL query tree item have been modified to define this function properly. The parser is setting this position for all parsed items. The test class/main function "TestGetPositionInAllADQLObject" aims to check that after a parsing all items really have a defined position.

ADQL: Completely change/improve Joins management, and particularly NATURAL JOIN and USING JOIN. Now, every joined columns are represented by a DBCommonColumn instance which has a given table coverage.
Before, there was a problem while using at least 3 or 4 NATURAL JOINs. Bug raised by Menelaos Perdikeas (ESAC).