Add NNC class

This class provides the raw non-neighboring connections data as read
from the deck and/or added using the addNNC method.
The NNC data is currently not processed. I.e. multiple NNC connection
between the same cell can exist side by side.

testdata/integration_tests/NNC/NNC.DATA

@@ -0,0 +1,37 @@
+RUNSPEC
+
+OIL
+GAS
+WATER
+
+
+DIMENS
+   10 10  1  /
+
+GRID
+
+DXV
+10*1000.0
+/
+
+DYV
+10*1000.0
+/
+
+DZ
+100*20.0 
+/
+
+TOPS
+100*10
+/
+
+NNC
+1 1 1 2 1 1 0.5 /
+1 1 1 1 2 1 1.0 /
+/
+
+NNC
+1 1 1 2 1 1 0.5 /
+1 2 1 1 2 1 1.0 /
+/

testdata/integration_tests/NNC/noNNC.DATA

@@ -0,0 +1,28 @@
+RUNSPEC
+
+OIL
+GAS
+WATER
+
+
+DIMENS
+   10 10  1  /
+
+GRID
+
+DXV
+10*1000.0
+/
+
+DYV
+10*1000.0
+/
+
+DZ
+100*20.0 
+/
+
+TOPS
+100*10
+/
+