pooling layer base implementation

[ghost]

As a first step for implementing CNN, this PR introduce new layer class PoolingLayer. This layer extends LayerBase, and include implementation for isLearnable(), getOutputShape() and generateParameterGradient(). This PR include some modifications in LayerParameter, which enable poolingLayer to make use of its new parameters. Also, PoolingLayerConfigurationBuilder class and PoolingLayerParameterInitializer class are added for configuration of pooling layer. neural_network.proto is modified to take inputs for pooling layer. Concrete implementation for feedForward() and backPropagate() will be made in later PRs.

[beomyeol]

@seyounglee95 I'll take a look. However, test build failed since NullPointException is thrown during LayerParameterArrayCodecTest. Please fix this :)

[beomyeol]

@seyounglee95 This looks good. I finished my pass and left some comments. Please check them :)

[ghost]

@beomyeol Thank you for your review. I fixed referring to your comments. Please take a look.

[beomyeol]

@seyounglee95 I finished another pass and left some comments.

[ghost]

@beomyeol Thank you for your review. I made some modifications. Please take a look.

[ghost]

@beomyeol I pushed a commit referring to your comment. Please check it.

[ghost]

@beomyeol I fixed comments. Thank you :D

[ghost]

@beomyeol Thank you for comments. I fixed declaration and comments.

[beomyeol]

@seyounglee95 Looks good. I finished my pass. @jsjason, can you take a look?

[jsjason]

@beomyeol @seyounglee95 Yes, I'll do a pass. I'll merge this right away (given that I don't see any problems), since this PR has been open for a long time.

[jsjason]

@seyounglee95 Great. I'll merge this after you're done checking my comments.

[ghost]

Thank you for review. Please take a look.

[jsjason]

Great. I'll merge this.