Added caching decorator (Parallel-in-Time#554)

* Implemented caching wrapper for spectral helper

* Added test for caching decorator

Author: brownbaerchen

pySDC/helpers/spectral_helper.py

@@ -11,6 +11,8 @@
 def cache(func):
     """
     Decorator for caching return values of functions.
+    This is very similar to `functools.cache`, but without the memory leaks (see
+    https://docs.astral.sh/ruff/rules/cached-instance-method/).
 
     Example:
 

pySDC/tests/test_helpers/test_spectral_helper.py

@@ -749,7 +749,7 @@ def test_identity_matrix_ND(nx, ny, nz, variant, bx, useMPI=False, **kwargs):
 
 
 @pytest.mark.base
-def test_cache():
+def test_cache_decorator():
     from pySDC.helpers.spectral_helper import cache
     import numpy as np