Implement a backend selecting mechanism

python/dune/perftool/generation/__init__.py

 from __future__ import absolute_import
 
+from dune.perftool.generation.backend import (backend,
+                                              get_backend,
+                                              )
+
 from dune.perftool.generation.counter import get_counter
 
 from dune.perftool.generation.cache import (cached,

python/dune/perftool/generation/backend.py

+""" Some decorators to implement a backend selection mechanism """
+
+_backend_mapping = {}
+
+
+def backend(name):
+    def _dec(func):
+        assert(name not in _backend_mapping)
+        _backend_mapping[name] = func
+        return func
+
+    return _dec
+
+
+def get_backend(name):
+    return _backend_mapping[name]