Imagery: add libsvm based imagery classification

[marisn]

A pair of modules revealing most of features of libsvm to GRASS. Libsvm is a popular library used in many machine learning programs (including scikit-learn). Contrary to some add-ons, this is a pure C implementation thus bypassing Python and depending on presence of libsvm.

Missing features:

• ability to use a precomputed kernel
• ability to provide weights for classifying unbalanced data
• parallel processing for value prediction/estimation

This is still a WIP as some rough edges still need to be ironed out.