Fix endings processing; cleanup of whitespaces and comments in tests

[zolaemil]

In 880bd7d I converted all indentations to spaces; So soft tabs have been used in the JS source files consistently, but it's been less consistent what we used in the HTML files. I think it's best to keep it simple and use soft tabs everywhere; unless there's a good reason to do otherwise...?

Fixes #50

[raffazizzi]

👍 I prefer soft tabs with two spaces in general.

[aerhard]

Hi, I've got one question: When I added partial ending support, I only allowed endings as siblings of section - that's because I was using https://music-encoding.org/sampleCollection/encodings/multiple_sectionsI.xml as a sample file; @zolaemil -- you now changed this so ending is only allowed as a child of section. Generally, your solution is more plausible to me; intuitively I would consider endings as parts of sections rather than additional material added after a section. But do you think it would make sense to allow both (ending as sibling as well as child) in order to support rendering MEI files like the one above?

[zolaemil]

Yeah I changed it because it seems to be more consistent with out current approach to section in general. That is, section is the container of everything we render. I think this question therefore relates to the more general question of how to treat multiple sections and mdivs; you touch on this in #43