Create class `ParametricPlot`

Create a new class ParametricPlot to store all parameters related to parametric plotting.

An instance of ParametricPlot should be created when reading a YAML file. For example:

simulation:

  mode: parametric

  parametric:
    parametric_mode: sequential
    parameters:
      - key: pipeline.photon_generation.illumination.arguments.level
        values: numpy.unique(numpy.logspace(0, 6, 100, dtype=int))

  outputs:
    output_folder:  'examples/PTC/output' 
    parametric_plot:                       <--+
      x:  'detector.image.mean'               |
      y:  'detector.image.std_deviation'      |
      plot_args:                              |
        title: 'CCD Photon Transfer Curve'    |
        xscale: 'log'                         |-- instance of `ParametricPlot`
        yscale: 'log'                         |
        xlim: [1., 2.e+6]                     |
        ylim: [1., 3.e+4]                     |
        grid: true                         <--+
 
  ...

Class ParametricPlot must be used when instantiating an Output class with parameter 'parametric_plot'.