merge_overlapping_streets improvements

* categories R0,S0,U0 have lower precedence now (this used to be wrong
  in the vicinity of Neuruppin, where a short part of RE6 was plotted
  as disused)
* don't add empty names (there were a couple of rbahn names with just
  a comma, or an additional trailing comma)
data update (after fixing merge_overlapping_streets)
GPS::GpsmanData: flat_track works for combined trk+wpt files

Previously, flat_track failed because the waypoint chunks had no
associated track. Now track-less chunks are simply ignored.

With test case.
GPS::GpsmanData: as_gpx does not emit useless <cmt> elements

If a comment field in a gpsman file is detected as a date-time value,
then only the <time> element is emitted, but not anymore the
equivalent <cmt> element. The old behavior can be restored by setting
the autoskipcmt option to a false value.

Adapted the gps-gpsmandata.t test. Also, this test now tests with "is"
instead of "like" in some places, using a better XPath query with an
explicit index.
gps-gpsmandata.t: test for parsing Name fields from tracks
GPS::GpsmanData::as_gpx(): dump <name> for trk and rte chunks

Previously, only wpt had <name> elements.

With test cases.
GPS::GpsmanData: harmless code fix in convert_DDD_to_DDD
new test script gps-gpsmandata-static.t