Add Base.conj for AbsSimpleNumFieldElem

[SoongNoonien]

As discussed with @fieker, @lgoettgens and @joschmitt on Friday at the OSCAR workshop I've added a Base.conj method for AbsSimpleNumFieldElem. The main purpose of this is to provide an easy way to conjugate cyclotomic numbers interactively (for actual implementations complex_conjugation should be better). After looking at the code I'm not sure why checking the parent field should be necessary. I think we can leave all the checks to complex_conjugation.

[codecov]

Codecov Report

Attention: Patch coverage is 76.47059% with 8 lines in your changes missing coverage. Please review.

Project coverage is 75.87%. Comparing base (d9b7787) to head (c1c70c4).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/NumField/NfAbs/Conjugates.jl	76.47%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1622      +/-   ##
==========================================
+ Coverage   75.78%   75.87%   +0.08%     
==========================================
  Files         361      361              
  Lines      113695   113693       -2     
==========================================
+ Hits        86164    86261      +97     
+ Misses      27531    27432      -99     

Files with missing lines	Coverage Δ
src/NumField/NfAbs/Conjugates.jl	83.98% <76.47%> (+0.09%)	⬆️

... and 38 files with indirect coverage changes

[thofma]

I think it is reasonable to have this, thanks. How about we cache the result of complex_conjugation(K) using @attr? Then we can remove the !!! note.

[SoongNoonien]

I'm not very familiar with this @attr. Do you mean something like this: @attr NumFieldHom{AbsSimpleNumField, AbsSimpleNumField, MapDataFromAnticNumberField{AbsSimpleNumFieldElem}, MapDataFromAnticNumberField{AbsSimpleNumFieldElem}, AbsSimpleNumFieldElem} function complex_conjugation(...?

[thofma]

I noticed that we cannot use @attr, since it does not work with keyword arguments. I pushed a commit adding the caching and merge once everything is green.