Maya: AssetManager: 1. Added abc_start_frame_offset and abc_start_frame_offset_enabled parameter. This parameter is a pre-roll BUT it is written out, which is needed for the cloth sim. Its defaults are enabled and 50 frames offset. It is controlable in the asset_manager_globals and by the alembic menu. 2. Added MayapyExportProcess class in the asset_manager_mayapy module, which replaces the old linear scripted export process.