Resolve "Clean up plotting of Results"

Closes #45 (closed)

• Dispersion dataframes now store frequencies in Hz
• Plotting of results has been ported to plotly, with much better interactivity than mpl
• all mode indices can be provided as single values or ranges and in desired units.
• when plotting, axes can be scaled using SIPrefix, e.g. provided eigen_result.plot(fscale='G') will plot the dispersion in units of GHz. Available prefixes are 'P', 'T', 'G', 'M', 'k', 'h', 'da', '', 'd', 'c', 'm', 'u', 'n', 'p', 'f'.

Important: The 3D plotting still needs to be cleaned up under the hood, but since everything works fine, this will be done in a later version.