Add arguments for PythonLayer

[semisight]

This pull request adds a way for PythonLayers (#1703) to accept arguments through prototxt files.

Instead of loading files or other processing in setup(), this allows one to pass parameters through the 'arguments' string in PythonParameter. The receiver can then process the string however they choose: load through json.loads(), comma separated values, etc.

This pull request is a simpler alternative to #2001, without exception handling changes.

Example class:

class MultLayer(caffe.Layer):
    def setup(self, bottom, top):
        self.mult_factor = int(self.arguments)

    def reshape(self, bottom, top):
        top[0].reshape(bottom[0].num, bottom[0].channels, bottom[0].height,
            bottom[0].width)

    def forward(self, bottom, top):
        top[0].data[...] = self.mult_factor * bottom[0].data

    def backward(self, top, propagate_down, bottom):
        if propagate_down[0]:
            bottom[0].diff[...] = self.mult_factor * top[0].diff

Example addition to prototxt:

layer {
  name: "python1"
  type: "Python"
  bottom: "prev1"
  top: "curr1"
  python_param {
    module: "multlayer"
    layer: "MultLayer"
    arguments: "10"
  }
}

[BlGene]

A simple parameter string seems to be a good solution. I would still add a comment recommending a particular way of passing parameters ( JSON seems suitable ) as the default, just to minimize unnecessary work of dealing with different formats.

@longjon does this sufficiently solve the second last point of #1376?

[shelhamer]

This is adding an arguments property to all Layers and not just PythonLayer.

[shelhamer]

Thanks for contributing your own version of parameter passing to Python layers, but I'm closing this in favor of the original change by @tnarihi. The change here incorrectly introduces a new property for all layers.

See #2871 for the cherry-picking of the PythonLayer parameter string part of #2001 alone.