Tweaking code to find default location of the database

It turns out that this bit of code was correct, but hopefully now it's clearer.

It's still pretty confusing where this is set, and in what priority.
(eg. the .rmgrc config file). I'm not sure why the prior one seems to work in other places

Author: rwest

rmgpy/__init__.py

@@ -139,8 +139,8 @@ def reset(self):
         Reset all settings to their default values.
         """
         self.filename = None
-        working_dir = os.path.abspath(os.path.dirname(__file__))
-        self['database.directory'] = os.path.realpath(os.path.join(working_dir, '..', '..', 'RMG-database', 'input'))
+        rmgpy_module_dir = os.path.abspath(os.path.dirname(__file__))
+        self['database.directory'] = os.path.realpath(os.path.join(rmgpy_module_dir, '..', '..', 'RMG-database', 'input'))
         self.sources['database.directory'] = 'Default, relative to RMG-Py source code'
 
 # The global settings object