neuron_volumina_viewer: Refactored main so that it uses a configure file to determine the layers to read and which ones to sync. This config file is quite large, however, so it is worth revisiting how main and the config file can be changed to make the needed configure file smaller (possibly by allowing regex). Also, this resulted in requiring the synchronization of channels to be updated and put into a functor to work. Now, it is possible to chose which layers are synced through the config file, by grouping them. This also forces layers that should be in sync to be neighbors, which is a reasonable constraint. This fixes #26, fixes #27, and refixes #19. This also marks the completion of the milestone Volumina Viewer (Basic GUI), which closes #10.