Add utils.plot_parameter_difference and _parameter_difference_subplot

Takes 2 components and a string (name of a parameter) as input, and plots the
difference between the component's parameters in a figure.
Add docstring to plot_parameter_difference and _parameter_difference_subplot
Fix docstring error in plot_parameter_report
Add utils.plot_normalized_parameter and utils._parameter_normalized_subplot

-utils._parameter_normalized_subplot:
Takes a list of components and a parameter name, sums up all the
parameter values, and then plots each parameter normalized by this sum.
Useful for comparing the area in fine structure when the intensity
changes over for example a line scan.
-utils.plot_normalized_parameter:
Basically a "wrapper" for utils._parameter_normalized_subplot