Unit test for future changes on SGSparseVector.
Avoid copy/malloc in SGSparseVector::sort_feature() :
* changed qsort_index to sort inline
* changed compression to work in line (and filter zero-entries)
* added switch "stable_pointers" to prevent re-alloc
* added unit-tests for new behaviour (in previous commit)
Merge pull request #1467 from tklein23/sort_features_refactoring

Last bit of sort_features() refactoring