PERF: Parallel streamline was slow. Fixed.

vtkPStreamTracer was causing the creation of a new locator at
every iteration (task), which is very slow for cell locators
since they have to bin data every time. Fixed by removing
the reallocation and using one locator. The input data does not
change between iterations so recreationg the locator was totally
unnecessary.

Change-Id: I966f25caf2e05ba08355a426f3e31f1ac66517ef
Merge topic 'fix-locator-recreation' into master

f82238ed PERF: Parallel streamline was slow. Fixed.