Add and update examples. Add rake tasks.

Author: v0dro

Rakefile

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+require 'rspec/core/rake_task'
+require 'bundler/gem_tasks'

examples/mapflat_vs_flat_map/benchmark_plot_graph.png

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+require 'benchmark/plot'
+require 'matrix'
+require 'nmatrix'
+require 'nmatrix/atlas'
+
+sizes = [5, 10, 50, 100, 150, 200]
+
+Benchmark.plot sizes, title: "Matrix multiplication",
+  file_name: "matrix_multiplication" do |x|
+  x.report "NMatrix with ATLAS" do |size|
+    n = NMatrix.new([size,size], [1]*size*size, dtype: :float32)
+    n.dot(n)
+  end
+
+  x.report "Matrix" do |size|
+    n = Matrix[*[[1]*size]*size]
+    n * n
+  end
+end

examples/matrix_benchmarks.rb

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+require 'numo-narray'
+require 'nmatrix'
+