* fixed invalid read in SGSparseVector::sort_features(): Worked on realloc'ed memory.
* SGSparseVector::sort_features(): freeing allocated memory as soon as possible
* Introduced clone() method for SGSparseVector (similar to SGVector).  Added unit test.
Merge pull request #1412 from tklein23/sparse_fixes

SGSparseVector: fixed invalid reads in sort_features() and introduce clone() method + unit test