Support one-dimensional VTK output

[mdpiper]

Dan Parsons (d.parsons@hull.ac.uk) found that setting VTK output for HydroTrend caused his WMT run to fail. Here's the stack trace:

Traceback (most recent call last): 
File "/home/csdms/wmt/bin/wmt-slave.py", line 430, in main env=env) 
File "/home/csdms/wmt/bin/wmt-slave.py", line 398, in launch task.execute() 
File "/home/csdms/wmt/bin/wmt-slave.py", line 311, in execute self.run() 
File "/home/csdms/wmt/bin/wmt-slave.py", line 380, in run model.go(file='model.yaml') 
File "/home/csdms/wmt/internal/coupling/cmt/component/model.py", line 45, in go self._components[self.driver].go(self.duration) 
File "/home/csdms/wmt/internal/coupling/cmt/component/component.py", line 157, in go self.run(stop_time) 
File "/home/csdms/wmt/internal/coupling/cmt/component/component.py", line 176, in run self._events.run(stop_time) 
File "/home/csdms/wmt/internal/coupling/cmt/events/manager.py", line 34, in run event.run(self._timeline.time) 
File "/home/csdms/wmt/internal/coupling/cmt/events/printer.py", line 18, in run self._printer.write() 
File "/home/csdms/wmt/internal/coupling/cmt/portprinter/port_printer.py", line 60, in write self._printer.write(self._field) 
File "/home/csdms/wmt/internal/coupling/cmt/printers/vtk/vtu.py", line 205, in write tofile (field, file_name, **kwargs) 
File "/home/csdms/wmt/internal/coupling/cmt/printers/vtk/vtu.py", line 174, in tofile elements = get_vtu_elements (field, format=format, encoding=encoding) 
File "/home/csdms/wmt/internal/coupling/cmt/printers/vtk/vtu.py", line 124, in get_vtu_elements types = get_vtk_types (field) 
File "/home/csdms/wmt/internal/coupling/cmt/printers/vtk/vtu.py", line 106, in get_vtk_types types = [int (edge_count_to_type.get (offsets[0], VtkPolygon))] IndexError: index out of bounds

This is error is thrown for 1D output because offsets is empty; 2D output is fine.

Dan was able to work around this issue by using netCDF output instead of VTK.

This is likely low priority, since we should just use netCDF.