ZarrArray: add a getDimensionSeparator method

This allows the array's `DimensionSeparator` to be easily checked.

The corresponding test is also updated to use the new getter instead of
reflective access.

src/main/java/com/bc/zarr/ZarrArray.java

@@ -210,6 +210,10 @@ public ByteOrder getByteOrder() {
         return _byteOrder;
     }
 
+    public DimensionSeparator getDimensionSeparator() {
+        return _separator;
+    }
+
     public void write(Number value) throws IOException, InvalidRangeException {
         final int[] shape = getShape();
         final int[] offset = new int[shape.length];

src/test/java/com/bc/zarr/ZarrArrayTest_dimensionSeparator.java

@@ -91,11 +91,8 @@ public void tearDown() throws Exception {
     @Test
     public void openOldStyleZarrArrayAndDetectDimensionSeparator() throws IOException, NoSuchFieldException, IllegalAccessException {
         final ZarrArray array = ZarrArray.open(arrayPath);
-        final Object separator = TestUtils.getPrivateFieldObject(array, "_separator");
-        assertThat(separator)
-                .isNotNull()
-                .isInstanceOf(DimensionSeparator.class);
-        DimensionSeparator sep = (DimensionSeparator) separator;
+        DimensionSeparator sep = array.getDimensionSeparator();
+        assertThat(sep).isNotNull();
         assertThat(sep.getSeparatorChar()).isEqualTo("/");
     }
 
@@ -104,4 +101,4 @@ private void prepareOldStyleArrayHeaderFile(FileSystemStore store) throws IOExce
         final OutputStream os = store.getOutputStream(".zarray");
         TestUtils.deleteLineContaining("dimension_separator", is, os);
     }
-}
+}