QVT — When and where clause for mapping operations

Reusing the 'when' and 'where' clauses of the refined relation
of a mapping operation to represent the 'when' and 'where' clauses
of the mapping operation is semantically not very accurate since
there are some differencies. Should better have their own
representation.

Suggestion: add specifics MappingOperation::when and
MappingOperation::where associations.

(1) In Section 8.2.1.15, add the associations
MappingOperation::when and MappingOperation::where
with the following descriptions:
when : OclExpression [0..1]
The pre-condition or the guard of the operational mapping.
It acts as a pre-condition when the operation is called with strict
semantics, it acts as a guard otherwise (see MappingCallExp)
where : OclExpression [0..1]
The post condition for the operational mapping.
(2) In Section 8.2.1.15, in the introductory part, replaces
"The when clause in the refined relation acts ..." by
"The when clause acts ..."
and "The where clause in the refined relation always acts as
a post-condition " by "The where clause always acts as
a post-condition "