* Remove the line, leave only the scatter plot

Allow to invert x axis when plotting dHvA frequencies
  * add `invert_x: bool = False` argument to `plot_xy` function,
    which is called when executing `plot_frequency_workchain` or
    `plot_frequency_calc` functions.
This is useful when comparing different rotation paths if
at one of the paths the relevant angle is decreasing.

Small fix to avoid AiidaDeprecationWarning

… axes labels and figure title.

 - pass **kwargs to ax.scatter. This allows to change markers, color, marker size etc.
 - allow to scale/offset x axis by specifying `scale_x` or `x_offset` keywords to facilitate comparison.
 - `show_legend` keyword to enable/disable legend. Default is `True`