gh-447: make sure dz gives the same results as nbins (#448)

glass/observations.py

@@ -226,7 +226,7 @@ def fixed_zbins(
     if nbins is not None and dz is None:
         zbinedges = np.linspace(zmin, zmax, nbins + 1)
     elif nbins is None and dz is not None:
-        zbinedges = np.arange(zmin, zmax, dz)
+        zbinedges = np.arange(zmin, np.nextafter(zmax + dz, zmax), dz)
     else:
         msg = "exactly one of nbins and dz must be given"
         raise ValueError(msg)