moved SQLTableCrossJoin::getActiveRow() to the super class: SQLTableJoin.

The moved function: SQLTableCrossJoin::getActiveRow() simply joins
two row (elements in ItemGroup instances). This operation will
 be also used from other type of join. So this was moved to the
super class to be called from other type of join class such as
SQLTableInnerJoin. Since SQLTableCrossJoin doesn't have conditions,
it can use the super class's getActiveRow() as it is (without
override). Other class's getActiveRow() should have some code to
check the condition.  So SQLTableInnerJoin::getActiveRow() keeps
 as not-implemented. (i.e. we should implent it)