An optimization that prunes nested <g>-tags when they contain exactly
one <g> and nothing else (except whitespace nodes). This looks a bit
like `removeNestedGroups` except it only touches <g> tags without
attributes (but can remove <g>-tags completely from a tree, whereas
this optimization always leaves at least one <g> tag behind).
Closes: #215
Signed-off-by: Niels Thykier <niels@thykier.net>
