Add unit tests

[solarchemist]

This being our most mature R package, perhaps it is a good candidate for introducing unit tests.

The "R packages" book is a good place to start
https://r-pkgs.org/testing-basics.html

[solarchemist]

I think this package could benefit a lot from unit tests.
Be explicit about the kinds of UV-Vis spectra that this algorithm has been tested to work for.
A noisy spectrum, an obscured band edge, the different kinds of transitions, etc.