added assertion to meilib unit test
test summary
MeiDoc class to deal with rich MEI and its section views

- based on VariantMei and SingleVariantPathMei.
- deals with grouped alternatives encoded with <annot @plist>
renamed test cases
added slicing to MeiDoc
consistent var names in comment and string const
comments: small fix and removed TODO
added doc comment
removed debug log
removed obsolete classes, methods and objects
Merge branch 'refactor-meilib'
iterate through Arrays with integer

(because meitovexflow adds functions to Array prototype)
mark for removal (would change parameter list)
fix for section plane update