Replace subgroup by sub

[fingolfin]

Completely untested, but subgroup does not seem to be defined anywhere? Perhaps I missed something, or perhaps these functions are simply not covered by CI?

Motivated by https://github.com/oscar-system/Oscar.jl/issues/667

[thofma]

I think I used it in some project a while ago. I am sure I added it for a reason, but unfortunately without tests it seems.

[fingolfin]

I guess you renamed those in #396 and then perhaps due to some time overlap between PRs these additional uses crept in?

[codecov]

Codecov Report

Merging #576 (bc7e33b) into master (2a2ecb7) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #576      +/-   ##
==========================================
- Coverage   69.52%   69.51%   -0.02%     
==========================================
  Files         312      312              
  Lines       92059    92059              
==========================================
- Hits        64006    63994      -12     
- Misses      28053    28065      +12     

Impacted Files	Coverage Δ
src/NumFieldOrd/NfOrd/Ideal/Prime.jl	79.42% <0.00%> (-0.62%)	⬇️
src/NumFieldOrd/NfOrd/Clgp.jl	76.33% <0.00%> (-4.92%)	⬇️
src/Map/NfRelOrd.jl	88.29% <0.00%> (-2.13%)	⬇️
src/NumFieldOrd/NfOrd/Ideal/TwoElement.jl	57.03% <0.00%> (-1.45%)	⬇️
src/LocalField/neq.jl	74.74% <0.00%> (-1.02%)	⬇️
src/NumField/NfAbs/CompactRepresentation.jl	82.16% <0.00%> (-0.96%)	⬇️
src/Misc/Series.jl	46.70% <0.00%> (-0.55%)	⬇️
src/NumFieldOrd/NfOrd/Clgp/Saturate.jl	64.76% <0.00%> (-0.55%)	⬇️
src/NumField/NfAbs/MPolyAbsFact.jl	70.72% <0.00%> (-0.54%)	⬇️
... and 19 more

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