add lazy module importer

We will need to issue deprecation warnings when using this with
'kwant.continuum', so we have to roll our own so we can hook into
the attribute access. When we no longer need this feature we
can replace this with a simpler implementation using
'importlib.util.LazyLoader'.

kwant/_common.py

@@ -6,10 +6,12 @@
 # the file AUTHORS.rst at the top-level directory of this distribution and at
 # http://kwant-project.org/authors.
 
+import sys
 import numpy as np
 import numbers
 import inspect
 import warnings
+import importlib
 from contextlib import contextmanager
 
 __all__ = ['KwantDeprecationWarning', 'UserCodeError']
@@ -123,3 +125,26 @@ def get_parameters(func):
     takes_kwargs = any(i.kind is inspect.Parameter.VAR_KEYWORD
                        for i in pars.values())
     return required_params, default_params, takes_kwargs
+
+
+class lazy_import:
+    def __init__(self, module, package='kwant', deprecation_warning=False):
+        if module.startswith('.') and not package:
+            raise ValueError('Cannot import a relative module without a package.')
+        self.__module = module
+        self.__package = package
+        self.__deprecation_warning = deprecation_warning
+
+    def __getattr__(self, name):
+        if self.__deprecation_warning:
+            absolute_module = '.'.join((self.__package, self.__module))
+            msg = ("Accessing {0} without an explicit import is deprecated. "
+                   "Instead, explicitly 'import {0}'."
+                  ).format('.'.join((self.__package, self.__module)))
+            warnings.warn(msg, KwantDeprecationWarning, stacklevel=2)
+        relative_module = '.' + self.__module
+        mod = importlib.import_module(relative_module, self.__package)
+        # Replace this _LazyModuleProxy with an actual module
+        package = sys.modules[self.__package]
+        setattr(package, self.__module, mod)
+        return getattr(mod, name)