Ensure spatial indices are positive values

In the event that ti=-1, the `ei` attribute will be negative. While we want to capture the negative value for `ti`, all spatial indices ought to be positive.

Author: fluidnumerics-joe

parcels/field.py

@@ -1297,7 +1297,7 @@ def unravel_index(self, ei):
         """
         _ei = ei[self.igrid]
         ti = _ei // (self.grid.xdim * self.grid.ydim * self.grid.zdim)
-        _ei = _ei % (self.grid.xdim * self.grid.ydim * self.grid.zdim)
+        _ei = abs(_ei) % (self.grid.xdim * self.grid.ydim * self.grid.zdim)
         zi = _ei // (self.grid.xdim * self.grid.ydim)
         _ei = _ei % (self.grid.xdim * self.grid.ydim)
         yi = _ei // self.grid.xdim