Incompatible with case-insensitive filesystems

I'm using a Mac.

```
git clone git@github.com:nomisma/data.git

warning: the following paths have collided (e.g. case-sensitive paths
on a case-insensitive filesystem) and only one from the same
colliding group is in the working tree:

  'id/AE3_4.rdf'
  'id/ae3_4.rdf'
  'id/Ilistra.rdf'
  'id/ilistra.rdf'
```

(etc).  19 pairs of files collide.