Allow dependent variables to handle arithmetic operations 

**Describe the reason for the request**

Dependent variables cannot currently handle arithmetic, like multiplication: 
```

test = cp.as_dependent_variable(np.array([1,2,3,4,5]))
print(test*2)
TypeError: unsupported operand type(s) for *: 'DependentVariable' and 'int'

```
this functionality could be useful in some circumstances