second attempt to add the VCF exporter

The VCF exporter finally generates output that can be used in R.
Would it be safer to check the line count here rather than hashing and getting the hex digest? The number of lines in the header should be constant, call them `H` lines. Then, you should have one line per variant right? If so, then the total number of lines in the returned file should be `H + V` where `V` is the number of expected variants. You can get that number by running a query matching your `test_params` so something like:

```python
variant_count = Variant.objects.filter(....).count()
```

And then assert that `variant_count` is the same as the number of lines in the exporter VCF plus some `H` lines to account for the header VCF lines.