Plotting Classes

As discussed with @gsell we should implement base classes for plotting in order to avoid the if-statements

if not isinstance(ds, DatasetBase):
        raise TypeError("Dataset '" + ds.filename +
                        "' not derived from 'DatasetBase'.")
    
    if not ds.filetype == FileType.OPTIMIZER:
        raise TypeError(ds.filename + ' is not an optimizer dataset.')

Each dataset would also be derived from the appropriate plotter class. This would also simplify adding new methods and avoiding errors.