[Staging] Neon should check if clef has invalid @line

[yinanazhou]

Recent MEI files have clefs that have invalid @line attribute.

Expected behaviour:

• Neon should give notification about it and provide @mei:id
• Neon should have new MEI action that can fix it
• MEI encoding job in Rodan should be fixed regarding this

[yinanazhou]

Hi @JoyfulGen and @annamorphism, I noticed that Rodan produces clefs with invalid line. In MEI, the value must be in the range between 1 and the number of lines on the staff. But I've found straight outta OMR files have clefs with line equals to 0 or 5. I wonder what is the best way to fix this. We have two options for the MEI action:

1. remove the invalid clefs
2. modify the value, so that all values below 1 would be changed to 1, and all values above 4 would be changed to 4

Which one would you prefer?