On RTD make trident use dummy config and ion files to avoid failure.

trident/config.py

@@ -263,5 +263,16 @@ def verify(save=False):
     print("Now let's science!")
     print("")
 
-ion_table_dir, ion_table_file = parse_config()
-ion_table_filepath = os.path.join(ion_table_dir, ion_table_file)
+# Each time Trident is imported, we determine the settings from the config
+# file or try to create a config file.  But don't do this on readthedocs, or
+# it will fail in the build. In readthedocs environment, just set a dummy
+# filepath so readthedocs can parse the docstrings OK.
+
+on_rtd = os.environ.get('READTHEDOCS') == 'True'
+if on_rtd:
+    ion_table_dir = trident_path()
+    ion_table_file = '__init__.py'
+    ion_table_filepath = os.path.join(ion_table_dir, ion_table_file)
+else:
+    ion_table_dir, ion_table_file = parse_config()
+    ion_table_filepath = os.path.join(ion_table_dir, ion_table_file)