(1) Convert methods which don't use self to static
(2) Update documentation
(3) Fix workspace deletion not updating plot
(4) Fix behaviour of FDA interface such that the plot is updated after a fft calculation and the labels correctly represent the fft component being plotted.

Also this commit fixes some failing unit tests in fitting and the Muon context

Moves the appropriate unit tests from plotting_widget to the newly refactored plot_widget

Various plot fitting improvements:

(a) Multiple fits no longer plotted
(b) Fits stay in results table