Changed "layup" to be a global variable and a short comment to explain it

Author: Little-Ryugu

src/layup/utilities/data_processing_utilities.py

@@ -59,6 +59,10 @@
 
 logger = logging.getLogger(__name__)
 
+# default Cache directory name where the layup auxiliary files are stored when layup bootstrap is ran
+# eg on Mac ~/Library/Caches/layup
+cache_dir_name = "layup"
+
 
 def write_fallback_obscodes():
     """Decompress the observatory-codes file bundled with layup to a plain JSON
@@ -356,7 +360,7 @@ class LayupObservatory(SorchaObservatory):
     A wrapper around Sorcha's Observatory class to provide additional functionality for Layup.
     """
 
-    def __init__(self, cache_dir=str(pooch.os_cache("layup"))):
+    def __init__(self, cache_dir=str(pooch.os_cache(cache_dir_name))):
         """Create an instance of the LayupObservatory class.
 
         Parameters