Fix file permissions

[mguaypaq]

Various data files were incorrectly marked as executable. To correct this, I ran:

$ find derivatives/ -type f -perm /+x -exec chmod -x {} +
$ git commit -a

Afterwards, I checked that all files and directories have uniform permissions:

$ find . -name .git -prune -o -name .github -prune -o -exec stat -c %A {} + | sort -u
drwxr-xr-x
-rw-r--r--

Since I did this in parallel with #138, I'm sure there will be merge conflicts, so I'll mark this as a draft for now.