Fix ixseps* indices for divertor-leg-only grid

It makes most sense to set ixseps1 and ixseps2 so that the divertor-leg
grid is treated as a SOL-only grid, where neither y-boundary connects to
anything.

hypnotoad/core/mesh.py

@@ -3860,6 +3860,12 @@ def writeGridfile(self, filename):
                     # separatrices in the same radial location
                     ixseps2 = ixseps1
 
+            if "single_region" in self.equilibrium.user_options and "divertor" in self.equilibrium.user_options.single_region:
+                # Need to hack topology indices to get something sensible for
+                # single_region case.
+                ixseps1 = -1
+                ixseps2 = -1
+
             f.write("ixseps1", ixseps1)
             f.write("ixseps2", ixseps2)
             f.write("jyseps1_1", jyseps1_1)