Prune by location (rather than by size)

[HaleySchuhl]

Is your feature request related to a problem? Please describe.
The pcv.morphology.prune function currently works by checking the length of each secondary segment (primary segments are not currently eligible for pruning since they are responsible for keeping the skeleton as a fully connected line graph).

Describe the solution you'd like
Filter on y-axis (or x-axis) and remove segments completely below OR partially below a user-defined threshold.

Additional context
Helpful for tillering plants. Helps to drop non-biologically relevant segments from consideration.

The lower portion of a plant will often display leaf occlusion/overlap artifacts, while the top part of the plant continues to be unhindered from trait extraction. Consider only recently emerged leaves at the top of the image.