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)