BUG: gradingDescriptor stream input ignores negative expansion (Fix #1841)

- mostly only tripped when using fractional edge grading, since this
  is where the list reading is used. Overseen in commit 7da0b5be.

src/mesh/blockMesh/gradingDescriptor/gradingDescriptor.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2015 OpenFOAM Foundation
-    Copyright (C) 2019 OpenCFD Ltd.
+    Copyright (C) 2019-2020 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -129,6 +129,11 @@ Foam::Istream& Foam::operator>>(Istream& is, gradingDescriptor& gd)
         is.readEnd("gradingDescriptor");
     }
 
+    if (expansionRatio_ < 0)
+    {
+        expansionRatio_ = 1.0/(-expansionRatio_);
+    }
+
     is.check(FUNCTION_NAME);
     return is;
 }