Redefinition context

[glebbelov]

This issue tracks implementation of checks ensuring that redefinitions consider expression context. Example:

minimize Obj:
   if x==1 then 1;

s.t. Con:
   numberof 1 in (x) >= 1;

When IfThen is flattened, it produces conditional expression for x==1 in negative context. Then, only when numberof is redefined, it adds positive context to x==1. Make sure this happens before x==1 is redefined.

Tasks.

1. Implement checks that context is not extended beyond that used for redefinition.
2. Or, alternatively, enable redefinition context extension. This is likely not necessary as long as we don't support model modification.
3. Topological redefinition graph sorting to stay correct.