File Path Handling: call for consistency

[HaleySchuhl]

Is your feature request related to a problem? Please describe.
In the case of pcv.io.read_dataset the argument is named source_path, but in the case of pcv.transform.checkerboard_calib the argument is named img_path. In two other plantcv functions, pcv.transform.merge_images, pcv.visualize.pixel_scatter_plot, pcv.visualize.time_lapse_video, and pcv.segment_image_series uses slightly different logic where the arguments is paths_to_imgs, paths_to_imgs, img_list, and imgs_paths respectively and in all these cases, the user is expected to provide a list of filepaths rather than putting all input images into a single directory.

Describe the solution you'd like
I personally think it's more user friendly to have the function take a directory rather than using pcv.io.read_dataset prior to running functions that take more than one image as input. I also think img_dir is succinct while descriptive but definitely open to discussion.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context, sample data, or code relevant to the feature request here.