Moved cache functionality from lbmpy to pystencils

cache.py

+try:
+    from functools import lru_cache as memorycache
+except ImportError:
+    from backports.functools_lru_cache import lru_cache as memorycache
+
+try:
+    from joblib import Memory
+    diskcache = Memory(cachedir="/tmp/lbmpy", verbose=False).cache
+except ImportError:
+    # fallback to in-memory caching if joblib is not available
+    diskcache = memorycache(maxsize=64)
+
+
+# joblibs Memory decorator does not play nicely with sphinx autodoc (decorated functions do not occur in autodoc)
+# -> if this script is imported by sphinx we use functools instead
+import sys
+calledBySphinx = 'sphinx' in sys.modules
+if calledBySphinx:
+    diskcache = memorycache(maxsize=64)