Move landmark_reference_pt_dist to the homology submodule

[nfahlgren]

Describe your changes
Relocate the pcv.landmark_reference_pt_dist function to pcv.homology.landmark_reference_pt_dist.

Type of update
Is this a: update

For the reviewer
See this page for instructions on how to review the pull request.

• PR functionality reviewed in a Jupyter Notebook
• All tests pass
• Test coverage remains 100%
• Documentation tested
• New documentation pages added to plantcv/mkdocs.yml
• Changes to function input/output signatures added to updating.md
• Code reviewed
• PR approved

[codecov]

Codecov Report

Merging #1199 (0bbc91b) into release-4.0 (8a6ae12) will not change coverage.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##           release-4.0     #1199   +/-   ##
=============================================
  Coverage       100.00%   100.00%           
=============================================
  Files              174       174           
  Lines             7694      7694           
=============================================
  Hits              7694      7694           

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)
...tcv/plantcv/homology/landmark_reference_pt_dist.py	100.00% <ø> (ø)
plantcv/plantcv/homology/__init__.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes