correction for MMG5_swapbin_int()

[mpotse]

This function can generate a compiler warning because it assumes MMG5_int is an 8-bytes type. The function was meant for i/o but is not used. I made it compile without warning and commented that for i/o purposes a function using MMG5_int (intended for internal use only) is probably not useful.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.29%. Comparing base (c48f68d) to head (cb028f0).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #322      +/-   ##
===========================================
- Coverage    50.30%   50.29%   -0.02%     
===========================================
  Files          177      177              
  Lines        47857    47858       +1     
  Branches     10355    10355              
===========================================
- Hits         24075    24069       -6     
- Misses       16050    16058       +8     
+ Partials      7732     7731       -1     

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