plog models perspective logarithm \( x\log(y/x) \)

### Syntax

```
f = plog(y,x);
f = x*log(y/x);
```

Note that the perspective logarithm automatically is detected in simple cases.

Perspective logarithm is implemented as a convexity aware callback operators, except when Mosek 9, SCS or ECOS are used and the exponential cone description is exploited in a graph representation.