multiplication of CyLPExpr by constant not commutative

When multiplying by a constant before or after a variable.sum() I get a wrong / good answer
```python
from cylp.cy.CyClpSimplex import CyClpSimplex

s = CyClpSimplex()
x = s.addVariable("x", 2)
y = s.addVariable("y", 2)
s += y >= 0
s += y.sum() - x.sum() * 3 <= -1 # ==> gives wrong solution x=[0,1]
# s += y.sum() - 3 * x.sum() <= -1 # ==> gives correct solution x=[0,0.3333]
s.objective = x.sum()
s.primal()
```
The two expressions `y.sum() - x.sum() * 3 <= -1` and `y.sum() - 3 * x.sum() <= -1` should represent the same constraint and give the same results but only the second gives the correct model.