Include surface as an object type

[vijaiaeroastro]

Surface is listed as an object type in the core spec.

But it is missing in the lib3mf.xml.
This PR introduces it.

Fixes #440
Closes 3MFConsortium/lib3mf_python#8

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.66%. Comparing base (59e82bc) to head (bc2915c).
⚠️ Report is 5 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #442   +/-   ##
========================================
  Coverage    59.66%   59.66%           
========================================
  Files           64       64           
  Lines        24646    24646           
========================================
  Hits         14705    14705           
  Misses        9941     9941           

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