Calculate section level for special sections in the API

[mojavelinux]

Calculate the section level for special sections in the API so that it doesn't leak into client code. Currently, the following code is showing up in the converters to handle what the parser should have dealt with:

def section_level(sec = self)
  @_section_level ||= (sec.level == 0 && sec.special) ? 1 : sec.level
end

You can see it as well in the built-in converters.

[mojavelinux]

To be more specific, special sections can be defined at level-0 in multi-part books. However, they are treated as level-1 sections. This shift should occur in the parser.

(Special sections are defined this way so that they can be siblings of parts instead of nested inside of them).