Rename subgroup -> sub

[thofma]

Closes oscar-system/Oscar.jl#667.

[codecov]

Codecov Report

Merging #396 (2e5be77) into master (bbf010e) will decrease coverage by 0.01%.
The diff coverage is 90.00%.

@@            Coverage Diff             @@
##           master     #396      +/-   ##
==========================================
- Coverage   66.16%   66.14%   -0.02%     
==========================================
  Files         297      297              
  Lines       89564    89564              
==========================================
- Hits        59259    59243      -16     
- Misses      30305    30321      +16     

Impacted Files	Coverage Δ
src/AlgAssAbsOrd/PIP.jl	17.44% <ø> (-0.76%)	⬇️
src/Deprecations.jl	4.54% <ø> (ø)
src/Grp/GenGrp.jl	66.96% <88.88%> (+0.76%)	⬆️
src/Grp/Morphisms.jl	85.27% <100.00%> (ø)
src/NumField/NfAbs/PolyFact.jl	75.51% <0.00%> (-8.35%)	⬇️
src/QuadForm/Herm/Lattices.jl	68.69% <0.00%> (-2.03%)	⬇️
src/AlgAss/AlgAss.jl	79.15% <0.00%> (-1.77%)	⬇️
src/NumField/NfAbs/Conjugates.jl	88.62% <0.00%> (-1.01%)	⬇️
src/Misc/nmod_poly.jl	61.83% <0.00%> (-0.79%)	⬇️
src/NumField/NfAbs/MPolyAbsFact.jl	71.62% <0.00%> (-0.76%)	⬇️
... and 13 more

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