Add more specificity of data recorded by `morphology.fill_segments`

[HaleySchuhl]

Is your feature request related to a problem? Please describe. Right now when someone inputs leaf objects and stem objects into the function pcv.morphology.fill_segments, all area measurements are stored in a single list. This is less than ideal since this doesn't allow users to differentiate between which of those observations are leaf area vs which one is the combined stem area.

Describe the solution you'd like The behavior does not need to change when people input one source of objects. However, when the parameter stem_objects is used, then the area of this contour should get saved as it's own unique variable.

Describe alternatives you've considered

Additional context