Fix mapslices

[meggart]

Should solve #546 and other mapslices-related issues.

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 69.80%. Comparing base (e68e036) to head (0913d51).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/helpers.jl	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #553      +/-   ##
==========================================
+ Coverage   68.35%   69.80%   +1.44%     
==========================================
  Files          14       14              
  Lines        2111     2113       +2     
==========================================
+ Hits         1443     1475      +32     
+ Misses        668      638      -30     

Files with missing lines	Coverage Δ
src/DAT/xmap.jl	53.77% <100.00%> (+3.55%)	⬆️
src/helpers.jl	82.75% <85.71%> (+39.42%)	⬆️

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