Support python 3.9/3.10; Improve documentation for add_factor; Bump up version for new release

[StannisZhou]

Will make a new release after this PR is merged so that people can use the package under python 3.9/3.10.

@nathanielvirgo I made some updates to the documentation for add_factor. Feel free to leave comments if the description is still not clear.

Resolves #115. Resolves #117

[codecov-commenter]

Codecov Report

Merging #120 (f8b81fb) into master (f7f3e3f) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #120   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            8         8           
  Lines          640       671   +31     
=========================================
+ Hits           640       671   +31     

Impacted Files	Coverage Δ
pgmax/fg/graph.py	100.00% <ø> (ø)
pgmax/__init__.py	100.00% <100.00%> (ø)
pgmax/fg/nodes.py	100.00% <0.00%> (ø)
pgmax/fg/groups.py	100.00% <0.00%> (ø)
pgmax/bp/bp_utils.py	100.00% <0.00%> (ø)

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[NishanthJKumar]

LGTM! Thanks for doing this!