FORMAT

Hi, 

I have encountered the following issue that I wanted to let you know of.

When I use the following command:

```python
df = allel.vcf_to_dataframe('myfile.vcf', fields=['calldata/*', 'variants/*'])
```

The FORMAT field is not parsed. However it is parsed when I use:

```python
callset = allel.read_vcf('myfile.vcf', fields=['calldata/*', 'variants/*'])
```
