[R/load&save] change slicer numbers. closes #203

[JanMarvin]

This is a fix for the current slicer behaviour. And another call for a check when saving that all relationships are available.

[codecov-commenter]

Codecov Report

Merging #205 (c56adbd) into main (805a669) will decrease coverage by 0.01%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main     #205      +/-   ##
==========================================
- Coverage   82.66%   82.65%   -0.02%     
==========================================
  Files          36       36              
  Lines        7397     7398       +1     
==========================================
  Hits         6115     6115              
- Misses       1282     1283       +1     

Impacted Files	Coverage Δ
R/class-workbook.R	75.61% <0.00%> (-0.04%)	⬇️
R/wb_load.R	95.25% <100.00%> (ø)

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