Set startseed for streamlines to None = middle of the domain.

[danlipsa]

No description provided.

[doutriaux1]

@danlipsa should we add a test for this?

[danlipsa]

I looked to modify the tests we have to not set the startseed. I turns out that startseed is already not set.
The previous default was [0, 0, 0] which was the startseed.
Now the default is None which sets startseed to the middle of the domain which is [0,0,0]. So we end-up with the same streamlines.
This this new code path is already tested by the tests we have.

[danlipsa]

Fixes
#269