fix idxlist arg + start adding shID slct

[cbyrohl]

Fixes #29

[codecov]

Codecov Report

Patch coverage: 10.65% and project coverage change: -1.50% ⚠️

Comparison is base (9e148e0) 56.72% compared to head (578264e) 55.22%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #82      +/-   ##
==========================================
- Coverage   56.72%   55.22%   -1.50%     
==========================================
  Files          54       52       -2     
  Lines        4850     4978     +128     
  Branches      961      986      +25     
==========================================
- Hits         2751     2749       -2     
- Misses       1890     2020     +130     
  Partials      209      209              

Files Changed	Coverage Δ
tests/customs/test_arepo.py	12.05% <8.69%> (-2.58%)	⬇️
src/scida/customs/arepo/dataset.py	31.51% <11.71%> (-2.85%)	⬇️

... and 2 files with indirect coverage changes

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