Implemented the viscosity correlation for n-Heptane of Assael, JPCRD, 2014 (http://scitation.aip.org/content/aip/journal/jpcrd/43/2/10.1063/1.4875930) and testing data

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>

dev/fluids/n-Heptane.json

@@ -394,6 +394,57 @@
         ], 
         "type": "polynomial"
       }
+    },
+    "viscosity": {
+      "BibTeX": "Assael-JPCRD-2014-Heptane", 
+      "dilute": {
+        "C": 2.1357e-08, 
+        "a": [
+          0.33974, 
+          -0.49396, 
+          0.08050
+        ], 
+        "molar_mass": 0.100202, 
+        "molar_mass_units": "kg/mol", 
+        "t": [
+          0, 
+          1, 
+          3
+        ], 
+        "type": "collision_integral"
+      }, 
+      "epsilon_over_k": 426.118, 
+      "epsilon_over_k_units": "K", 
+      "higher_order": {
+        "hardcoded": "n-Heptane"
+      }, 
+      "initial_density": {
+        "b": [
+          -19.572881, 
+          219.73999, 
+          -1015.3226, 
+          2471.01251, 
+          -3375.1717, 
+          2491.6597, 
+          -787.26086, 
+          14.085455, 
+          -0.34664158
+        ], 
+        "t": [
+          0, 
+          -0.25, 
+          -0.5, 
+          -0.75, 
+          -1.0, 
+          -1.25, 
+          -1.5, 
+          -2.5, 
+          -5.5
+        ], 
+        "type": "Rainwater-Friend"
+      }, 
+      "sigma_eta": 0.61362e-9, 
+      "sigma_eta_units": "m"
     }
   }
 }