nodes consisting of only a single whitespace can occur and thus may not be purged

closes #3

Although we once changed this and placed the comments right where there are
encoded, this leads to problems with the blocklevel element processing in the
digital view of the website: rdgMarkers seem to disappear because they are
sorted into the editorial comments.

Therefore we decided to revert the changes once made. This will have to be
considered for the print edition also.

In order to improve performance we only determine the rdg position during
the HTML creation in bdn-SADE/modules/critical.xqm since it's not necessary
elsewhere.