[ENH] Fit-Check added to NiftiLabelsMasker

Contrary to NiftiMasker, NiftiLabelsMasker.transform() does not currently check whether fit() of the current instance has already been called. Added exception for this case.
Slight niggle: you separated this comment from the line it is talking about, namely 

```python
fmri11_img_r = masker11.inverse_transform(signals11)
```