GAMS writer fails for power nested in a log

The functionality to selectively replace `**` with `power()` fails in an edge case when `**` is nested within a `log` function, e.g. `log(x ** 2)`. In `replace_power()`, the `assert` that `arg` is bound by `(` and `)` fails because the arg is bound by `log(` and `)`. This is possibly also true with other similar functions, e.g. `exp()`.

@gseastream 