doc is updated for ppm_gmm_* inside ppmnumerics. all the other files inside ppmnumerics should follow the rules described in [wiki:Coding_Guide] and take ppm_gmm_* as example. 

git-svn-id: https://ppm.inf.ethz.ch/svn/ppmnumerics/branches/ngtopo/libppmnumerics@539 7c7fe9aa-52eb-4d9e-b0a8-ba7d787348e9