Add a description with a prototype

[dumoulin]

Actually, a prototype and a variable doesn't have any metadata, particularly a description. It could be usefull, for the user to be able to annotate its variables.

[dumoulin]

Proposed solution :

• add String description attributes in Prototype class
• serialize the string as a sub-element of the prototype :

  <variable name="myVariable" type="string">
     <description class="string">Description here !</description>
     <value class="string"></value>
  </variable>
  

[dumoulin]

Do the same with StructureNode

[dumoulin]

Fix in r956