set_mode to CPU in ArgMaxLayerTest constructor to avoid random Travis
failures.
Add NetState message with phase, level, stage; NetStateRule message with
filtering rules for Layers.
Incorporate NetState{,Rule} into Solver/Net.  Net::FilterParam
includes/excludes layers based on whether the NetState meets each
layer's NetStateRule(s).
Add INPUT layer so deploy nets can be incorporated as well.
Use unified train/test nets in examples.