[core] add low-level retro-compatibility for attribute changes

First version of retrocompatibility, allowing to load files referencing removed or type-incompatible attributes.
* add node_factory to centralize node instantiation
* discard invalid attributes (i.e. not part of the node description anymore or with incompatible value type) when loading a file
* raise on unknown nodes
* add 'core.exception' module to declare Meshroom's exception types

meshroom/core/exception.py

@@ -0,0 +1,22 @@
+#!/usr/bin/env python
+# coding:utf-8
+
+
+class MeshroomException(Exception):
+    """ Base class for Meshroom exceptions """
+    pass
+
+
+class GraphException(MeshroomException):
+    """ Base class for Graph exceptions """
+    pass
+
+
+class UnknownNodeTypeError(GraphException):
+    """
+    Raised when asked to create a unknown node type.
+    """
+    def __init__(self, nodeType):
+        msg = "Unknown Node Type: " + nodeType
+        super(UnknownNodeTypeError, self).__init__(msg)
+        self.nodeType = nodeType