Use global_z in the merscope coordinates

[quentinblampey]

As noticed in this issue, the MERSCOPE reader provides the x and y columns in PointsModel.parse, but not z.

The GLOBAL_Z key was already existing, but not used.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 49.54%. Comparing base (7cccdd1) to head (d7616b8).
⚠️ Report is 23 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #332      +/-   ##
==========================================
+ Coverage   49.06%   49.54%   +0.47%     
==========================================
  Files          26       26              
  Lines        2686     2721      +35     
==========================================
+ Hits         1318     1348      +30     
- Misses       1368     1373       +5     

Files with missing lines	Coverage Δ
src/spatialdata_io/readers/merscope.py	23.88% <ø> (ø)

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