Add boundary property on Surface

[paulromano]

This PR adds the ability to set the boundary property on a surface. Not all codes are able to take advantage of this but it does look like DAGMC has code recognizing boundary:Reflecting and boundary:Vacuum properties.

[pshriwise]

Very small comment from me but otherwise looks good @paulromano!

[pshriwise]

🚀

[gonuke]

One small edit to a comment to make it clear how general the group method is. Otherwise, looks good to me.

I see a value in generalizing some of these group operations in the future, but won't request it here :)

[codecov]

Codecov Report

❌ Patch coverage is 96.15385% with 1 line in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@1af3754). Learn more about missing BASE report.
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pydagmc/dagnav.py	96.15%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #53   +/-   ##
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  Coverage        ?   97.09%           
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  Files           ?        2           
  Lines           ?      448           
  Branches        ?       52           
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  Hits            ?      435           
  Misses          ?        9           
  Partials        ?        4           

Flag	Coverage Δ
unittests	97.09% <96.15%> (?)

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[pshriwise]

@gonuke brought your docstring fix in. Ready to merge in my eyes. Punch it in when you're ready plz.

[gonuke]

LGTM - thanks @paulromano