Hubungan antara modal sosial dan modal fisik dengan kesejahteraan petani sawit mandiri : studi di Desa Aek Batu, Kec. Torgamba, Kab. Labuhanbatu Selatan, Sumatera Utara = Correlation between social capital and physical capital with walfare of self developing oil palm smallholders : study at Aek Batu Village, Torgamaba subdistrict, South Labuhanbatu, North Sumatera province, Republic of Indonesia / Dwi Zatmiko