JNLPBA Dataset
## Describe the bug
A clear and concise description of what the bug is.

## Steps to reproduce the bug
```python
# Sample code to reproduce the bug
```

## Expected results
The dataset loading script for this dataset is incorrect. This is a biomedical dataset used for named entity recognition. The entities in the [script](https://github.com/huggingface/datasets/blob/master/datasets/jnlpba/jnlpba.py#L81-L83) are: O, B, and I.  The correct entities from the original data file are: 

['O',
 'B-DNA',
 'I-DNA',
 'B-RNA',
 'I-RNA',
 'B-cell_line',
 'I-cell_line',
 'B-cell_type',
 'I-cell_type',
 'B-protein',
 'I-protein']

## Actual results
The dataset loader script needs to include the following NER names:

['O',
 'B-DNA',
 'I-DNA',
 'B-RNA',
 'I-RNA',
 'B-cell_line',
 'I-cell_line',
 'B-cell_type',
 'I-cell_type',
 'B-protein',
 'I-protein']

And the [data](https://github.com/huggingface/datasets/blob/master/datasets/jnlpba/jnlpba.py#L46) that is being pulled has been modified from the original dataset and does not include the original NER tags.

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version:
- Platform:
- Python version:
- PyArrow version:
