New function to clear the cache (#437)

* Fixes an issue where passing a new attributes to IDF.simulate would fail to create a new cache

* reformat

* Added clear_cache method

- deletes the settings.cache_folder (files and sub-directories)
- Import from archetypal directly

archetypal/utils.py

@@ -775,3 +775,10 @@ def signif(x, digits=4):
         return x
     digits -= math.ceil(math.log10(abs(x)))
     return round(x, digits)
+
+
+def clear_cache():
+    """Clear the cache."""
+    import shutil
+
+    shutil.rmtree(settings.cache_folder)