Merge pull request #1080 from JasonRuonanWang/dataman-doc

DataMan for WAN

***************

Coming soon...

The DataMan engine is designed for data transfers over the wide area network. To use this engine, you can either specify it in your xml config file, with tag ``<engine type=DataMan>`` or set it in your application code:

.. code-block:: c++

 adios2::IO datamanIO = adios.DeclareIO("ioName");

 datamanIO.SetEngine("DataMan");

 adios2::Engine datamanReader = datamanIO.Open(filename, adios2::Mode::Write);

On the reader side you need to do instead:

.. code-block:: c++

 adios2::IO datamanIO = adios.DeclareIO("ioName");

 datamanIO.SetEngine("DataMan");

 adios2::Engine datamanReader = datamanIO.Open(filename, adios2::Mode::Read);

.. note::

 The DataMan engine currently does not support data staging within a cluster.

The DataMan engine does not accept any parameters. However, users are allowed to specify the following transport parameters:

1. **Library**: the underlying network / socket library used for data transfer.

2. **IPAddress**: the IP address of the host where the writer application runs.

3. **Port**: the port on the writer host that will be used for data transfers.

4. **Timeout**: the timeout in seconds to wait for every send / receive.

============= ================= ================================================

 **Key**       **Value Format**  **Default** and Examples

============= ================= ================================================

 Library           string        **ZMQ**

 IPAddress         string        **127.0.0.1**, 22.195.18.29

 Port              integer       **12306**, 22000, 33000

 Timeout           integer       **5**, 10, 30

============= ================= ================================================