Reverted MSL to 3.2.1.

This is for #24.
OpenModelica is using 3.2.1. Version 3.2.1 also fixes various uses of Modelica that are not according to the standard.
Updated UsersGuide.

This is for #21. The image was moved to the Fluid/UsersGuide directory (to reflect the class in which it is used). The description has been moved to the section 'Implementation' as most users, except for developers, may not need this information.