Update read geometry

[kdiers]

This PR changes the magic number that is used to assess the validity of presumed FreeSurfer surface files.

Previously, the computation of the magic number involved the application of the bit-shifting operator; however, this does not return identical results on all platforms. We therefore we disable it and return / compare the first three bytes of the file separately.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 34.89%. Comparing base (2de1c3f) to head (f577204).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
lapy/_read_geometry.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #64      +/-   ##
==========================================
- Coverage   34.91%   34.89%   -0.02%     
==========================================
  Files          14       14              
  Lines        2366     2367       +1     
  Branches      297      299       +2     
==========================================
  Hits          826      826              
- Misses       1456     1457       +1     
  Partials       84       84              

Flag	Coverage Δ
unittests	34.89% <0.00%> (-0.02%)	⬇️

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