Add SaveEntries() and LoadEntries(). (#233)

These allow saving and loading non-persistent entries in the persistent
file format.

src/main/java/edu/wpi/first/networktables/NetworkTable.java

@@ -264,6 +264,27 @@ public final class NetworkTable {
     return path;
   }
 
+  /**
+   * Save table values to a file.  The file format used is identical to
+   * that used for SavePersistent.
+   * @param filename  filename
+   * @throws PersistentException if error saving file
+   */
+  public void saveEntries(String filename) throws PersistentException {
+    inst.saveEntries(filename, pathWithSep);
+  }
+
+  /**
+   * Load table values from a file.  The file format used is identical to
+   * that used for SavePersistent / LoadPersistent.
+   * @param filename  filename
+   * @return List of warnings (errors result in an exception instead)
+   * @throws PersistentException if error saving file
+   */
+  public String[] loadEntries(String filename) throws PersistentException {
+    return inst.loadEntries(filename, pathWithSep);
+  }
+
   @Override
   public boolean equals(Object o) {
     if (o == this) {

src/main/java/edu/wpi/first/networktables/NetworkTableInstance.java

@@ -935,6 +935,29 @@ public final class NetworkTableInstance {
     return NetworkTablesJNI.loadPersistent(m_handle, filename);
   }
 
+  /**
+   * Save table values to a file.  The file format used is identical to
+   * that used for SavePersistent.
+   * @param filename  filename
+   * @param prefix    save only keys starting with this prefix
+   * @throws PersistentException if error saving file
+   */
+  public void saveEntries(String filename, String prefix) throws PersistentException {
+    NetworkTablesJNI.saveEntries(m_handle, filename, prefix);
+  }
+
+  /**
+   * Load table values from a file.  The file format used is identical to
+   * that used for SavePersistent / LoadPersistent.
+   * @param filename  filename
+   * @param prefix    load only keys starting with this prefix
+   * @return List of warnings (errors result in an exception instead)
+   * @throws PersistentException if error saving file
+   */
+  public String[] loadEntries(String filename, String prefix) throws PersistentException {
+    return NetworkTablesJNI.loadEntries(m_handle, filename, prefix);
+  }
+
   private final ReentrantLock m_loggerLock = new ReentrantLock();
   private final Map<Integer, Consumer<LogMessage>> m_loggers = new HashMap<>();
   private Thread m_loggerThread;

src/main/java/edu/wpi/first/networktables/NetworkTablesJNI.java

@@ -169,6 +169,9 @@ public final class NetworkTablesJNI {
   public static native void savePersistent(int inst, String filename) throws PersistentException;
   public static native String[] loadPersistent(int inst, String filename) throws PersistentException;  // returns warnings
 
+  public static native void saveEntries(int inst, String filename, String prefix) throws PersistentException;
+  public static native String[] loadEntries(int inst, String filename, String prefix) throws PersistentException;  // returns warnings
+
   public static native long now();
 
   public static native int createLoggerPoller(int inst);