fix is_abelian (and some more)

[fieker]

fixes Simon's problem on slack:

julia> F, a = cyclotomic_field(7,:a)
(Cyclotomic field of order 7, a)

julia> P,x = polynomial_ring(F,:x)
(Univariate polynomial ring in x over F, x)

julia> FG,dd = number_field(x^6 + (133486969//288a^5 - 223759151//432a^4 - 223759151//432a^3 + 133486969//288a^2 - 232474865//288)x^3 - 840539241479//13824a^5 - 18036715089631//9216a^4 - 18036715089631//9216a^3 - 840539241479//13824*a^2 - 7320065966297//9216) (Relative number field of degree 6 over F, _$)

julia> is_abelian(FG)
ERROR: Number field element not in the order
Stacktrace:
...

The same problem (plus some more) also would have prevented maximal_abelian_subfield(FG, of_closure = true)
to have been correct

The runtime of those examples is a bit high for testing (always)

[codecov]

Codecov Report

Attention: Patch coverage is 36.36364% with 7 lines in your changes missing coverage. Please review.

Project coverage is 75.84%. Comparing base (c8604b9) to head (3d33af5).
Report is 36 commits behind head on master.

Files with missing lines	Patch %	Lines
src/RCF/conductor.jl	36.36%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1629      +/-   ##
==========================================
+ Coverage   75.72%   75.84%   +0.11%     
==========================================
  Files         358      361       +3     
  Lines      113693   113702       +9     
==========================================
+ Hits        86095    86235     +140     
+ Misses      27598    27467     -131     

Files with missing lines	Coverage Δ
src/RCF/conductor.jl	77.03% <36.36%> (-0.41%)	⬇️

... and 98 files with indirect coverage changes