Join clause of lookups should support predicates

[Seddryck]

At the moment, it's possible to define the join clause of a lookup operator as a mapping or using. These two conditions support the equality between a column from the candidate table and a column of the reference table. It's possible to define several columns mapping or using to specify that multiple equalities between columns are required.

It would be convenient to let the user define a more generic condition than just equality. The predicate is especially suited for this.

The following syntax could be used

<join>
  <predicate operand="any-column-from-candidate-table">
    <greater-than>[any-column-from-reference-table]</greater-than>
  </predicate>
</join>

Combination of predicates could also be used.

<join>
  <combination operator="and">
    <predicate operand="any-column-from-candidate-table">
      <greater-than>[any-column-from-reference-table]</greater-than>
    </predicate>
    <predicate operand="any-column-from-candidate-table">
      <less-than>[any-other-column-from-reference-table]</less-than>
    </predicate>
  </combination>
</join>

This feature is relying on the implementation of #472