fixed 'append' saving option for par1 vs par2 case

[sofia-calgaro]

In the previous PR, it was left out the case "saving" : "append" when we are looking at "variation": true

[sofia-calgaro]

Re-tested following 'append' cases:

• 1 par
  1. "variation": false and limits=[None, None]
  2. "variation": true and limits=[A%, B%]!=[None, None]
• 2 pars, one vs another one
  1. "variation": false and limits_1=[None, None] , limits_2=[None, None]
  2. "variation": false and limits1=[A1%, B1_%] , limits2=[A2%, B2_%]

Plot info dictionary stored for each parameter in the final shelve object has the following entries:

dict_keys(['title', 'subsystem', 'locname', 'plot_style', 'time_window', 'resampled', 'range', 'std', 'unit', 'label', 'unit_label', 'limits', 'param_mean', 'parameter', 'variation'])

[sofia-calgaro]

Note: checked that the saved plot info dictionary is compatible with the previous version (all relevant keys for plotting are there and correct for each variation and number of parameters case)