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Fixed a small bug when resolving layers. Updated dependency versions, and bumped project version number

Renamed VariableSymbol to ParameterSymbol, introduced layer variable declarations and changed IOSymbol to VariableSymbol which now combines IO variables and layer variables

This reverts merge request !29

Updated pom: cnn-arch version changed to 0.2.3-SNAPSHOT and cnn-train version changed to 0.2.4-SNAPSHOT.
Minor changes in pooling layers.