Rename void IO and add tee()

src/main/java/com/widen/tabitha/RowReader.java

@@ -19,7 +19,7 @@ public interface RowReader extends Iterable<Row>, Closeable
      *
      * Closing has no effect on this row reader and is always re-usable.
      */
-    RowReader EMPTY = Optional::empty;
+    RowReader VOID = Optional::empty;
 
     /**
      * Create a row reader from an array of rows.

src/main/java/com/widen/tabitha/RowWriter.java

@@ -14,7 +14,37 @@ public interface RowWriter extends Closeable
      *
      * Closing has no effect on this row writer and is always re-usable.
      */
-    RowWriter NULL = row -> {};
+    RowWriter VOID = row -> {};
+
+    /**
+     * Creates a new row writer that writes rows to multiple destinations.
+     *
+     * @param rowWriters The writers to write to.
+     * @return The new row writer.
+     */
+    static RowWriter tee(RowWriter... rowWriters)
+    {
+        return new RowWriter()
+        {
+            @Override
+            public void write(Row row) throws IOException
+            {
+                for (RowWriter rowWriter : rowWriters)
+                {
+                    rowWriter.write(row);
+                }
+            }
+
+            @Override
+            public void close() throws IOException
+            {
+                for (RowWriter rowWriter : rowWriters)
+                {
+                    rowWriter.close();
+                }
+            }
+        };
+    }
 
     /**
      * Writes a row to the output.

src/test/groovy/com/widen/tabitha/DataFrameTest.groovy

@@ -6,7 +6,7 @@ class DataFrameTest extends Specification {
     def "test is streaming"() {
         setup:
         def inMemory = new DataFrame()
-        def streaming = DataFrame.streaming(RowReader.EMPTY)
+        def streaming = DataFrame.streaming(RowReader.VOID)
 
         expect:
         !inMemory.isStreaming()

src/test/groovy/com/widen/tabitha/RowReaderTest.groovy

@@ -5,7 +5,7 @@ import spock.lang.*
 class RowReaderTest extends Specification {
     def "empty reader"() {
         setup:
-        def reader = RowReader.EMPTY
+        def reader = RowReader.VOID
 
         expect:
         !reader.read().isPresent()

src/test/groovy/com/widen/tabitha/RowWriterTest.groovy

@@ -3,10 +3,10 @@ package com.widen.tabitha
 import spock.lang.*
 
 class RowWriterTest extends Specification {
-    def "null is always writable"() {
+    def "void is always writable"() {
         setup:
         100.times {
-            RowWriter.NULL.write(Mock(Row.class))
+            RowWriter.VOID.write(Mock(Row.class))
         }
     }
 }