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[mpotse]

Fixed libmmg2d.h in the same way as libmmg3d.h:

• a \brief for every function
• grammar and spelling fixes
• added a general introduction
• separated notes for developers from those for users Also fixed a few more things in libmmg3d.h These are still relatively quick fixes, there is a lot more that can be improved. I left some FIXMEs for myself or others to figure out later, e.g. for functions that are declared but not implemented or documented.

[Algiane]

Thanks!