layer definition reorganization and documentation
- split out neuron, loss, and data layers into own header files
- added LossLayer class with common SetUp checks
- in-progress concise documentation of each layer's purpose
Split all loss layers into own .cpp files
Cosmetic change in prep for data layer work
fwd/back math docs for neuron layers
Lil’ more docstring, and cosmetic change in EuclideanLossLayer
Cosmetic change in ConcatLayer
Incorporated Evan’s comments for neuron layers
Merge pull request #376 from sergeyk/layer_reorg

Layer definitions and declarations re-organization and documentation
merged with latest