Most pre-processing functions and process_and_segment_image expect 3 dimensions, but percentile_threshold seems to expect 4

[ellenemerson]

Most of the functions seem to expect images of 3 dimensions (height, width, channel), but percentile_threshold seems to expect 4, with the first dim being batch.

The documentation for histogram_normalization also references a batch dimension, but I don't think the code actually expects it.