Store fixed node positions in files

[curran]

When the nodes are positioned (fixed), their locations and the fact that they are fixed should be stored somewhere. This could be in the graph files themselves, but in that case the coordinates would be wiped out each time the graph files get generated. Better to store the fixedness and coordinates in separate files.

Once stored, they should be read by the client as the initial setting.

[curran]

Related d3/d3-force#35