Add basic unit test for StreamingDenseFeatures reading
Fix Example<T> in ParseBuffer.h
instead of trying to store the example as SGVector<T> in an Example<T>
rather store it as T* as T can be SGSparseVectorEntry.
Use SG_REALLOC instead of SG_MALLOC in StreamingAsciiFile when reading
sparse vectors, this way enabling true buffering in ParseBuffer
Add example_reading_from_features to StreamingDenseFeatures unit test