Case of absence of sym and/or name tag in gpx file

In case the sym and/or name tags are not present in the GPX file
appropriate measures are taken.
Also, if the divetime is absent in the GPX, the present device
time is taken as the divetime.
Previously as the timestamp was set to -1, the import showed improper
result.

Signed-off-by: Venkatesh Shukla <venkatesh.shukla.eee11@iitbhu.ac.in>