Remove default demand and network files

- Remove the default .dat file / cached network JSON from
  stats-config.json. Make each variant use its own .dat
  file (which already existed but weren't being used).
- Create two extra copies of the demand UFM file and add these
  to the x3 and x5 variants in the stats-config. These are
  identical to the x1 demand file, but allow us to stop having
  demand files in the defaults section.

stats-config.json

@@ -7,7 +7,6 @@
             "cached": "rules.json"
         },
         "demand": {
-            "file": "ExmouthRandMatrix.UFM",
             "args": {"filetype": "ufm", "userclasses": ["car"]}
         },
         "output": {
@@ -32,6 +31,9 @@
             "output": {
                 "file": "Exmouth_Network.UFS",
                 "cached": "output_x1congestion.json"
+            },
+            "demand": {
+                "file": "ExmouthRandMatrix.UFM"
             }
         },
         "x3congestion": {
@@ -42,6 +44,9 @@
             "output": {
                 "file": "Exmouth_Network_x3Congestion.UFS",
                 "cached": "output_x3congestion.json"
+            },
+            "demand": {
+                "file": "ExmouthRandMatrix_x3Congestion.UFM"
             }
         },
         "x5congestion": {
@@ -52,6 +57,9 @@
             "output": {
                 "file": "Exmouth_Network_x5Congestion.UFS",
                 "cached": "output_x5congestion.json"
+            },
+            "demand": {
+                "file": "ExmouthRandMatrix_x5Congestion.UFM"
             }
         }
     }