include raxml-format partition files

[roblanf]

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