Update the plotterparameter schema to associate it to a plotter

Right now there's no way to tell via the naming scheme or the plotterparameter JSON which plotter the parameter is associated to. The easiest way would be to add a field, say "plotter", which has the associated plotter object's name. See beat.cmdline#20 (closed) for the discussion that led up to this.

The idea could be something like what I suggested in the other thread:

{
  "plotter": "plot/bar/1",
  "grid": true,
  "title": "DET ISO/IEC 19795-1:2006",
  "title-fontsize": 12,
  "det": true,
  "legend-loc": "best",
  "width": 800,
  "height": 600,
  "dpi": 120,
  "legend-fontsize": 8,
  "xlim-left": 0,
  "xlim-right": 40,
  "ylim-bottom": 0,
  "ylim-top": 40,
  "axis-fontsize": 10
}

The "plotter" field stores the reference to the associated plotter.

However, it might be best to use a field name that the user couldn't accidentally use, such as #plotter.

@samuel.gaist @flavio.tarsetti @andre.anjos what do you all think? This is sorta blocking time-sensitive work so it'd be better to decide quickly.