support CIFAR-10, CIFAR-10 datasets

[hatappi]

add CIFAR-10, 100 datasets.

Usage

CIFAR1-10

> require 'datasets'
> c = Datasets::Cifar.new
> c.metadata
# => #<struct Datasets::Metadata name="CIFAR-10", url="https://www.cs.toronto.edu/~kriz/cifar.html", licenses=nil, description="CIFAR-10 is 32x32 image datasets">
> c.each do |r|                                                                                                                                                        
    p r.data
    # => [59, 43, 50, 68, 98, 119, 139, 145, 149, 143, .....]
    p r.label
    # => 6
  end 

CIFAR1-100

> require 'datasets'
> c = Datasets::Cifar.new(class_num: 100)
> c.metadata
# => #<struct Datasets::Metadata name="CIFAR-100", url="https://www.cs.toronto.edu/~kriz/cifar.html", licenses=nil, description="CIFAR-100 is 32x32 image datasets">
> c.each do |r|                                                                                                                                                        
    p r.data
    # => [71, 74, 75, 76, 78, 81, 81, 78, ....]
    p r.label
    # => 23
  end 

[hatappi]

@kou Please review my PullRequest.

[kou]

Thanks. I've merged and am working on some improvements. I'll write summary to this after the work.

[kou]

• Removed trailing spaces.
• Changed to use test data instead of the original data in test because the original data is too large to run tests on Travis CI.

[kou]

I'll do the followings:

• Change to use InvalidArgument instead of raw raise.
• Remove set_ from set_type because set_ is redundant. This context is "dataset". So we don't need to specify "set" in this context.
• Change to export coarse label in CIFAR-100 dataset. It'll be useful information.

[kou]

It may be better that we keep Record#data as byte string instead of an array of integer and Record#pixels returns an array of integer like the followings:

class Record < Struct.new(:data, :label)
  def pixels
    data.unpack("C*")
  end
end

[kou]

Record#to_gdk_pixbuf, Record#to_narray and so on may be useful.

[kou]

It may be better that we use Dataset::CIFAR instead of Cifar because the original dataset name is "CIFAR".

[kou]

e.g.: HTTP is better than Http for "HTTP".

[kou]

The usage in this pull request description is useful information. So we should put it to README.md or document comment in the source.

[hatappi]

@kou Thank you for comment. I will do the following.

1. Cifar => CIFAR
2. wirte README.md about CIFAR-10, 100
3. Record#pixels
4. set_type to type