handle get coord bounds for an axis with size=1 (#1030)

* handle get coord bounds for an axis with size=1

* fix comment

Author: haileyajohnson

cdm/core/src/main/java/ucar/nc2/dataset/CoordinateAxis1D.java

@@ -972,10 +972,18 @@ private boolean makeBoundsFromAux() {
   private void makeEdges() {
     int size = (int) getSize();
     edge = new double[size + 1];
-    if (size < 1)
+    if (size < 1) {
       return;
-    for (int i = 1; i < size; i++)
+    }
+    if (size == 1) {
+      // if the axis size is one, edges are the same as the coord
+      edge[0] = coords[0];
+      edge[1] = coords[0];
+      return;
+    }
+    for (int i = 1; i < size; i++) {
       edge[i] = (coords[i - 1] + coords[i]) / 2;
+    }
     edge[0] = coords[0] - (edge[1] - coords[0]);
     edge[size] = coords[size - 1] + (coords[size - 1] - edge[size - 1]);
     isContiguous = true;