Plot comparison results

[gevtushenko]

This PR adds visualization to the comparison script. Here's an example for:

// ...
  .add_int64_power_of_two_axis("elements", nvbench::range(12, 28, 4))
  .add_int64_axis("ratio", {0, 10});

When script is executed, one can specify the axis along which the visualization should be made:

nvbench_compare.py --plot-along elements ref.json cmp.json

During visualization, reference is plotted as dashed line and the noise is illustrated as an error band.