display bugs with overlapping correction notes

[russ]

when two notes are in the same place, and one note is elevated to a correction, we show a single "correction" bug on the note.

this is probably okay, but maybe worth reviewing.

here's an example:

 http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0005745

when two notes overlap (in this case, one note begins inside the other note's range), we show two bugs. in this case, if one note is elevated to a correction BOTH bugs are replaced by the correction bug. this is less okay.

here's an example:

 http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0005086