Add a readme file for building and using the Simulink interface

MBsysC/mbs_interface/mbs_simulink/ReadMe.md

+# Simulink interface to Robotran/MBsysC modules
+
+The Simulink/MBsysC interface enables to introduce a Robotran model
+into a Simulink block diagram using a S-function. This document explains
+ how to compile and how to use this interface.
+ 
+## Compiling the interface
+
+* First, the MBsysC momdules must be compiled already
+* Go to the `MBsysC/mbs_interface/mbs_simulink` folder
+* Create a *build* directory: `mkdir build`
+* Configure the Cmake project: `cmake ..`
+* Build the interface: `make`
+
+Those operations should produce a file *MBsysC_SF_dirdyn.mexa64* (the
+extension will change depending on the OS). This file must be compiled
+only once and can then be used for any project (assuming 
+FLAG_SEPARATE_BUILD was set to ON when building the project)
+
+
+## Compiling the project
+
+The project must be compiled as usual but **the FLAG_SEPARATE_BUILD 
+must be set to ON**.
+
+
+## Building a Simulink block diagram with a Robotran model
+
+* Add the `MBsysC/mbs_interface/mbs_simulink` folder to your matlab path. 
+  In matlab: `addpath('paht/to/MBsysC/mbs_interface/mbs_simulink')`
+* Create a new Simulink block diagram or poen an existing one
+* Insert a S-function block from the Simulink library (it can be found in the
+  Simulink/User-Defined Functions). Alternatively, you can copy/paste
+  the block from an existing diagram such as the examples project availabl in 
+  the `MBsysC/mbs_interface/mbs_simulink` folder.
+* Double-click on the S-function block and edit the propeties as follows:
+  * **S-function name**: type `MBsysC_SF_dirdyn` (the name of the compiled
+      file withou the extension)
+  * **S-function parameters**: enter the path to the *.mbs file surrounded
+       by single quotes
+  * **S-function modules**: keep the default setting
+* Connect the input/port of the block to the diagram:
+  * 1st input  is ...
+  * 2nd input  is ...
+  * 1st output contains the joint positions 
+  * 2nd output contains the joint velocities 
+  * 3rd output contains the joint accelerations 
+* Complete the Simulink diagram as desired
+* Run the simulation           
+
+## Adding input/output ports
+
+Custom input or output ports can be added to the block.
+
+* Define input/output (IO) by editing the *.mbs model with a text editor (this 
+can not be done in MBsysPad presently). Add a *user_variables* (with s!) section or
+complete the existing one (after the user_models section). Add as many
+user_variable (without s) section as IO number desired as illustrated
+by the following example:
+  ```xml
+  <user_variable>
+    <user_variable>
+      <varname>Fsusp</varname>
+      <type>out</type>
+      <size>4</size>
+    </user_variable>
+    <user_variable>
+      <varname>motorTorque</varname>
+      <type>in</type>
+      <size>1</size>
+    </user_variable><user_variable>
+  ```
+  * **varname** contains the name of the IO. The data will be 
+     readable/editable in the code via `mbs_data->userIO->varname`.
+  * **type** determines wether it is an input or an output  
+  * **size** defines the number of element in the IO. If size=1, the IO
+    will be a scalar (double). If size>1n the IO will be an array (double *) 
+
+* (Re-)generate the userIO.c and userIO.h files. This is done via
+  MBsysPad using the menu *Tools/Generate C-specific user files/All* 
+  (check that the *user_file_generation* utility in the 
+  `MBsysC/mbs_app` folder has been compiled before)
+  
+* Rebuild your project (the Simulink interface must NOT be rebuilt)
+
+* Reload the simulink diagram (or simply edit the properties of the 
+  S-function block). The new ports will then be visible.
+
+## Status
+
+* Simulation of several different model in the same diagram: OK
+* Platform
+  * Linux: OK
+  * Windows and MacOS: not tested
+* Type of building
+  * Separate build: OK  
+  * Monolithic build: not tested
+* Realtime features
+  * Not working. Should be implemented differently  
+* To be done
+  * improve integration with existing function (compute_model is duplicated)
+  * improve user interface (cf JFC work in MBsysLab/mbs_simulink)