Fix wrong integer conversion when computing xpmax in norver function

[Algiane]

As mesh->xpmax, mesh->xp and mesh->npmax are MMG5_int (since the adding of int64_t integers support), we have to cast the double value computed into a MMG5_int value and not a long long one.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 50.44%. Comparing base (88d9d1e) to head (53b5af3).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #281   +/-   ##
========================================
  Coverage    50.44%   50.44%           
========================================
  Files          177      177           
  Lines        47236    47236           
  Branches     10276    10276           
========================================
  Hits         23830    23830           
  Misses       15674    15674           
  Partials      7732     7732           

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[coprigent]

Thanks !