Changes to leaf phys vignette, still needs work on leaf spec figure

[lisahaber]

fd_leaf_spectrometry() missing columns for severity and treatment type, so boxplots figure can't really be rendered the way we want right now. But otherwise these changes should be good. Note that leaf morphology methods are described in the vignette since we will eventually add this type of data to the package, though there is currently no leaf_morphology function type in the package.

[atkinsjeff]

i will update the photo links to low res