feat: improve locally free class group

[thofma]

• compute the idempotents in the center

[codecov]

Codecov Report

Attention: Patch coverage is 73.33333% with 8 lines in your changes missing coverage. Please review.

Project coverage is 75.57%. Comparing base (a503506) to head (c1003f1).
Report is 1 commits behind head on master.

Files	Patch %	Lines
src/NumFieldOrd/NfOrd/ResidueRingMultGrp.jl	60.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1573      +/-   ##
==========================================
+ Coverage   75.56%   75.57%   +0.01%     
==========================================
  Files         357      357              
  Lines      113549   113574      +25     
==========================================
+ Hits        85798    85838      +40     
+ Misses      27751    27736      -15     

Files	Coverage Δ
src/AlgAssAbsOrd/LocallyFreeClassGroup.jl	92.47% <100.00%> (+0.14%)	⬆️
src/NumFieldOrd/NfOrd/ResidueRingMultGrp.jl	82.02% <60.00%> (-0.52%)	⬇️

... and 34 files with indirect coverage changes