Add (more) operator and conversion overloads to TFOutput

So we could write

```csharp
TFOutput result = 2.0;
result += 42.0 * g.Abs(x)));
```

instead of
```csharp
TFOutput result = g.Const(2.0);
result = g.Add(result, g.Mul(g.Const(42.0), g.Abs(x)));
```

In which ```g``` is of type ```TFGraph``` , and ```x``` of type ```TFOutput```  and both come from some where else in the graph/code.