 attempt to index field 'scalar_steps' (a nil value) when using add_scalar_dict

Hi,

I tried `add_scalar_dict` as following 

```
local foo = cc:create_experiment("foo")
local d = {}
d['bar'] = 3
d['hoo'] = 4
foo:add_scalar_dict(d)
```

but got the errors  

```
attempt to index field 'scalar_steps' (a nil value) 
```

not sure how to use that.


thanks