Added variables config maual + updated readme

doc/variables_config.md

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+# Configuring the variables and their parameters
+
+A programmer can configure the curves that are displayed in the graphical part.
+
+All this can be configured in the `variables_config.ini` file.
+
+## Functionnalities summary
+
+A programmer can :
+
+For a variable :
+- show a parameter which is not "value" or "target"
+- hide "value" or "target" parameter
+
+For a parameter :
+- specify the category of the corresponding curve
+- specify the color of the corresponding curve
+- specify the unit of the corresponding curve
+
+
+## File structure
+
+```ini
+[chart]
+<variable.parameter>=<options>
+```
+
+## Indicating a parameter
+
+To indicate a parameter, the programmer has first to indicate the SECOP name of the variable (case sensitive).
+If the options are concerning the `value` field, then nothing more has to be specified.
+- Example : `T_plato`
+
+In the other case, the programmer has to use a dot, followed by the name of the parameter (as it is in NICOS).
+- Example : `T_plato.raw`
+
+Finally, the equal sign must be used to then indicate the options and their values.
+- Example : `T_plato.target=<options>`
+
+## Giving the options
+
+### Using positionnal arguments
+
+One can give the different options for a parameter by using a comma separated value (without spaces around commas) list.   
+The order is `cat`, `color`, `unit`.
+
+- Example : `T_plato.raw=raw,#0000FF` This indicates that the `raw` parameter will be displayed in the raw category of its unit (found in NICOS if present), and will have a blue color.
+
+Be aware that this example is not the same as : `T_plato.raw=raw,#0000FF,` that indicates a unit equal to nothing.
+
+### Using keywords arguments
+
+One can indicate to which option a value is given, independently of its position in the comma separated list, by using keywords. This can be useful if we want to indicate only the unit without indicating/overwritting the category and/or the color.
+
+To use keywords : `<keyword>:<value>`
+
+Possible keywords are :
+- `cat` : specifies the category
+- `color` : specifies the color (6 digits hexadecimal color)
+- `unit` : specifies the unit
+
+Example : `T_plato=color:#0000FF,cat:other` This indicates that the `value` parameter of T_plato will be displayed in blue, in the "other" category of its unit (found in NICOS if present).
+
+### Using both
+
+Once a keyword argument is used for a parameter, it is then not possible to use positionnal arguments for this parameter.
+- Example : `T_plato=unit:K,#FF0000` is invalid, because no keyword is given after the use of `unit` for the other options.   
+
+Valid syntaxes for this example are `T_plato=unit:K,color:#FF0000` or `T_plato=color:#FF0000,unit:K`.
+
+## Special values and omissions
+
+If `cat` is `None`, then the associated parameter is not displayed. This value is case-sensitive. This can be used to explicitely hide `value` and/or `target` parameters of a variable.
+
+Omitting the category means that the parameter will be displayed in the default unit block.   
+Omitting the color means that an automatic color will be given for this parameter.   
+Omitting the unit means that it will be the one found in NICOS if present.   
+
+## Diverse examples
+
+- `T_plato=None` : the `value` parameter of T_plato is never displayed
+- `T_plato.raw=` : the `raw` parameter of T_plato will be displayed with an automatic color, will have its unit found in NICOS, and will be displayed in the default block corresponding to its unit