Add ability to only compute post analysis on subset of network

[risserlin]

Should be option for either known signatures or signature discovery as check box in the interface (on the known signatures should also do the filtereing step on just what is selected in the network.)

User selects a region of a larger EM and only wants to calculate signatures for that region.
In post analysis interface selects "use only selected nodes"
List potential signatures. (for signature discovery)
User selects those that interest them . (for signature discovery)
Score edges for the list of selected signatures only to the selected nodes in the network