Invalid MEI as test files

[fosfrancesco]

The following files:

• example_noMeasures_noBeams.mei
• example_noMeasures_withBeams.mei
• example_withMeasures_noBeams.mei
• example_withMeasures_withBeams.mei

are not valid MEI files (as double check, MEI companion raise a validation error when importing them) because they don't specify a namespace on the root MEI tag. As a result, this files are failing the test_load_score() test. This failing is hidden at the moment due to another unrelated problem in the load_kern function.

Where are these files from and why are they useful? I can see 2 solutions:

1. modify the MEI parser to accept these kind of files: if they are useful and from a good source (so we could expect more like that in the future)
2. delete the files: if they are not useful
3. modify the files: if they are useful but not from a good source

I can take care of that.