RvtkStatismo - update and some fixes

This morning I realised that I had introduced some issues with S4 classes: As R accepts the S3 class mesh3d as list, an object of class pPCA passes class-validation in R but NOT in Rcpp (on the C++ level). The ladder threw an exception when trying to import a model with a polygon mesh as representer. Thus, I had to add a virtual class called representer that now is a superclass of list and mesh3d. Now only the implementation of meshinfo seems to be missing (both in the wrapper and the S4 class specification), but it should not be a big problem to do that.

Documentation

I spent quite some time in making documentation more comprehensive (see updated manual).