Improve results plotting logic, bounded to pyCSEP refactoring .

[pabloitu]

There is current plot refactoring PR in pycsep (cseptesting/pycsep#263). Once this goes through, floatcsep needs to restructure the plotting logic to connect the Test function to a Plot function. Some ideas for improvement

• Always require the user to insert a plot function to a test config. From the Plot function signature/name, derive the logic (e.g. all forecasts, one forecast, all time windows, etc.)
• Define defaults for each test function. This implies functions are automatically passed, and only arguments are optional for the user. Any other plotting logic should be done through a plot hook.