Adjust output of GridOut::write_gnuplot() for 1d triangulations.

Adjust output of GridOut::write_gnuplot() for 1d triangulations.

The GridOut::write_gnuplot() function produced output
  for 1d meshes embedded in higher dimensional spaces that was
  invalid in that the lines showing individual cells were connected.
  While this is not wrong for singly connected 1d meshes, it leads to wrong
  results if the domain is not singly connected and not every cell is the
  right neighbor of the previous cell.