Import GraphML files correctly (Issue #149)

Looking at ImportContainerImpl and ImporterGraphML (and other importers), it appears that the problem is a problem of terminology. In GraphML, the 'default' type means that edges should be set to X by default, otherwise to whatever the specific setting is. In the importer, it appears to use the default as the *only* edge type, and forbids all other edge types.

Diving into the processor code, AppendProcessor.java checks the default edge type, and creates a graph according to that edge type, which does not appear to support other edge types. This would reflect the viewpoint of the importer. 

If you change the default edge type in the GraphML importer to MIXED, and set the edge type based on the default edge type, then the bug is fixed. Looking at other file formats, this adjustment should be made to other file formats that also support mixed representations.