Increase test coverage and documentation improvement

[ericpre]

As noticed in #33, it would be good to increase test coverage and documentation improvement. Here are some (in order of estimated difficulty):

• the functions to calculate inelastic mean free path are not documented in the user guide or the API reference.
• the estimate_variance_parameters
• some other functions in https://github.com/hyperspy/exspy/blob/main/exspy/misc/eels/tools.py may need to be reviewed to know it there are still necessary, for example: power_law_perc_area, rel_std_of_fraction and ratio are not tested and documented
• the Vignetting component is not test and its documentation is unclear.

[ericpre]

Labelling as "good first issue" because the first item in the list above should be for someone with basic knowledge of eels.

[ericpre]

@francisco-dlp, the two last items (eels tools and Vignetting component) comes from the early days. Could you help with these or provide guidance?