sources file created by ver_cli is broken

[raulcf]

To reproduce, start the ver_cli like this:
python aurum_cli -- --interactive

then, using the "ver" object:

1. create a sources file, ver.create_sources_file("testsources")
2. add a csv repo, ver.add_csv("testsources", "test", <PATH_TO_CSV_FILES>)
3. double check that the sources file looks correct, ver.inspect_sources_file("testsources")

At this point, if you run ver.profile(), the profiler will fail with:

[main] INFO ddprofiler.core.Main - Using /Users/raulcf/repos/projects/ver/.dsessions/sources/testsources.yml as sources file
java.lang.NullPointerException: Cannot invoke "java.util.List.iterator()" because "sources" is null

(running ddprofiler manually does not fix the problem)

I strongly suspect this is due to the format with which python is writing the YAML file to disk.