include raxml-format partition files

These look like this:

```
DNA, part1 = 1-100
DNA, part2 = 101-384
```

We can build these from the csv file, and we should name them similarly to the nexus format ones e.g. 

* partitions_raxml.txt
* loci_raxml.txt

etc

Then we should add tests to make sure they load OK in raxml-ng