Two io functions

[JorgeGtz]

Describe your changes
Started the subpackage I/O with two functions that were useful for the pixel scatter visualization:

• read_dataset - Returns a list of paths to the images in a given directory.
• random_subset - Returns a subset of the elements in a list.

Type of update
Is this a:

• New feature or feature enhancement

Associated issues
Discussion #724 and PR #810

Additional context

[codecov]

Codecov Report

Merging #823 (40e05ee) into 4.x (2c249a4) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               4.x      #823   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          151       154    +3     
  Lines         6527      6556   +29     
=========================================
+ Hits          6527      6556   +29     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
plantcv/plantcv/__init__.py	100.00% <ø> (ø)
plantcv/plantcv/io/__init__.py	100.00% <100.00%> (ø)
plantcv/plantcv/io/random_subset.py	100.00% <100.00%> (ø)
plantcv/plantcv/io/read_dataset.py	100.00% <100.00%> (ø)
plantcv/__init__.py	100.00% <0.00%> (ø)

[nfahlgren]

@JorgeGtz and @HaleySchuhl, not to overly complicate things, but what do you think about using the pcv.io.read_dataset method here in the timelapse video function in #762? That would just mean we would need to get this merged first and then refactor #762 somewhat.

[JorgeGtz]

@nfahlgren I can do the refactoring of #762 after this is merged.