Implement structure factor calculation

Author: JanLuca

varipeps/contractions/definitions.py

@@ -1,10 +1,14 @@
 from . import absorption
 from . import projectors
 from . import routine
+from . import structure_factor_absorption
+from . import structure_factor_routine
 
 from .routine import (
     calc_ctmrg_env,
     calc_ctmrg_env_custom_rule,
     CTMRGNotConvergedError,
     CTMRGGradientNotConvergedError,
 )
+
+from .structure_factor_routine import calc_ctmrg_env_structure_factor