introduce make_constant_restriction function, useful to build a restriction without adding it yet to the tree

it wasn't doing what it should have done (see cw.rset related_entity
implementation)

We actually can't use distinct if an attribute is selected  and we sort
on another attribute

      which may cause pb later (at sql generation time for instance)

fix rule order so 'HAVING (X op Y)' is now parseable while 'HAVING (1+2) op Y' isn't anymore parseable... At some point we should consider more advanced parsing system than yapps.

replace get_variable_variables mecanism which can't work with some UNION queries, with get_variable_indices (eg working on selection indices instead of variable names)