[Mod] move OffsetConverter module to vnpy.trader

vnpy/app/cta_strategy/engine.py

@@ -38,6 +38,7 @@ from vnpy.trader.constant import (
 from vnpy.trader.utility import load_json, save_json, extract_vt_symbol, round_to
 from vnpy.trader.database import database_manager
 from vnpy.trader.rqdata import rqdata_client
+from vnpy.trader.converter import OffsetConverter
 
 from .base import (
     APP_NAME,
@@ -50,7 +51,6 @@ from .base import (
     STOPORDER_PREFIX
 )
 from .template import CtaTemplate
-from .converter import OffsetConverter
 
 
 STOP_STATUS_MAP = {