Add cache to approximant finding

bilby/gw/utils.py

 import json
 import os
+from functools import lru_cache
 from math import fmod
 
 import numpy as np
@@ -701,6 +702,7 @@ def lalsim_SimInspiralTransformPrecessingNewInitialConditions(
     return SimInspiralTransformPrecessingNewInitialConditions(*args_list)
 
 
+@lru_cache(maxsize=10)
 def lalsim_GetApproximantFromString(waveform_approximant):
     from lalsimulation import GetApproximantFromString
     if isinstance(waveform_approximant, str):
@@ -799,6 +801,7 @@ def lalsim_SimInspiralChooseFDWaveform(
     return SimInspiralChooseFDWaveform(*args, waveform_dictionary, approximant)
 
 
+@lru_cache(maxsize=10)
 def _get_lalsim_approximant(approximant):
     if isinstance(approximant, int):
         pass