Increase default distance of `spectral_index` sub-package functions

[HaleySchuhl]

Start a discussion about anything you would like
The default distance of 20 was chosen as a default value within the pcv.spectral_index functions (mostly at random), but increasing to 30 would give our example data (e.g. the photosynthesis tutorial) the right amount of leniency to calculate most spectral indices under default settings. This has been discussed, but no issue was opened so I wanted to document the proposal here. I think this is an increase to user experience and generalizability, at the cost of specificity.

It would be interesting if we tested the difference in signal recorded while using the actual wavelength of band used in a given index changed, and thereby make recommendations about appropriate leniency per index.

Alternatively, we could consider the most common resolution of HSI sensors, which is often 10nm of smaller, so I could see an argument for leaving the default distance the same or even decreasing it.

[nfahlgren]

One thing to note is that it would be a breaking change because it would alter the default behavior that PlantCV4 users have been using, so it could not be implemented until PlantCV5 and with an adequate warning