Add NetState message with phase, level, stage; NetStateRule message with
filtering rules for Layers.
Add unit tests and skeleton code for Net/Solver filtering functionality.
Incorporate NetState{,Rule} into Solver/Net.  Net::FilterNet
includes/excludes layers based on whether the NetState meets each
layer's NetStateRule(s).
Use unified train/test nets in examples.
Merge pull request #734 from jeffdonahue/all-in-one-net

All-in-one nets