added doc for ssc

docs/user_guide/source/engines/ssc.rst

+**************************
+SSC Strong Staging Coupler
+**************************
+
+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 takes the following parameters:
+
+1. ``IPAddress``: No default value. The IP address of the host where the writer application runs.
+   This parameter is compulsory in wide area network data staging.
+
+2. ``Port``: Default **50001**. The port number on the writer host that will be used for data transfers.
+
+3. ``Timeout``: Default **5**. Timeout in seconds to wait for every send / receive operation.
+   Packages not sent or received within this time are considered lost.
+
+4. ``AlwaysProvideLatestTimestep``: Default **TRUE**.
+This is a boolean parameter that affects what
+of the available timesteps will be provided to the reader engine.  If
+AlwaysProvideLatestTimestep is **TRUE**, then if there are multiple
+timesteps available to the reader, older timesteps will be skipped and
+the reader will see only the newest available upon BeginStep.
+This value is interpreted by only by the DataMan Reader engine.
+If AlwaysProvideLatestTimestep is **FALSE**, then the reader engine
+will be provided with the oldest step that has not been processed.
+
+5. ``OneToOneMode``: Default **FALSE**. It is recommended that this parameter is set to TRUE when
+   there is only one writer process and only one reader process. This will explicitly tell both the
+   writer engine and the reader engine that it only needs to connect to a single writer or reader,
+   and thus saves the handshake overhead.
+
+=============================== ================== ================================================
+ **Key**                         **Value Format**   **Default** and Examples
+=============================== ================== ================================================
+ IPAddress                       string             **N/A**, 22.195.18.29
+ Port                            integer            **50001**, 22000, 33000
+ Timeout                         integer            **5**, 10, 30
+ AlwaysProvideLatestTimestep     boolean            **TRUE**, false
+ OneToOneMode                    boolean            **FALSE**, true
+=============================== ================== ================================================
+
+