Made the registration of d2o's variables robust against pre-initialization of...

Made the registration of d2o's variables robust against pre-initialization of the configuration object.

d2o/__init__.py

@@ -2,6 +2,7 @@
 
 from __future__ import division
 
+from config import configuration
 from distributed_data_object import distributed_data_object
 from d2o_librarian import d2o_librarian
 

d2o/config/d2o_config.py

@@ -29,8 +29,21 @@ variable_mpi_init_checks = keepers.Variable('mpi_init_checks',
                                             genus='boolean')
 
 # Construct the configuration object
-configuration = keepers.get_Configuration(
-                    'D2O',
-                    [variable_mpi_module,
-                     variable_default_distribution_strategy,
-                     variable_mpi_init_checks])
\ No newline at end of file
+configuration = keepers.get_Configuration('D2O')
+
+
+[configuration.register(vari) for vari in
+    [variable_mpi_module,
+     variable_default_distribution_strategy,
+     variable_mpi_init_checks]]
+
+
+# register the default comm variable as the 'mpi_module' variable is now
+# available
+variable_default_comm = keepers.Variable(
+                     'default_comm',
+                     ['COMM_WORLD'],
+                     lambda z: hasattr(dependency_injector[
+                                       configuration['mpi_module']], z))
+
+configuration.register(variable_default_comm)