Reconfigure dask futures and client management

[nfahlgren]

Describe your changes
This PR simplifies the pcv.parallel.multiprocess function to use the Dask client map function to submit jobs to the cluster. Map creates a futures object that can then be monitored with progress. The aim here is to properly populate the client cluster and shut it down upon completion to eliminate client errors that have been oberved.

Type of update
Is this a: update

Associated issues
#922

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 #958 (f77786d) into release-4.0 (457f072) will not change coverage.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##           release-4.0      #958   +/-   ##
=============================================
  Coverage       100.00%   100.00%           
=============================================
  Files              161       161           
  Lines             6860      6857    -3     
=============================================
- Hits              6860      6857    -3     

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/parallel/multiprocess.py	100.00% <100.00%> (ø)

[nfahlgren]

I have tested this using LocalCluster on my computer and HTCondorCluster on the cluster and it seems to avoid the client errors we observed sometimes before - but still functions the same