Introduce assure_reduced!(A::FinGenAbGroup, a::ZZMatrix)

[fingolfin]

Note that some of the replaced code was potentially wrong: it passed A.snf instead of A.hnf to reduce_mod_hnf_ur!. But I did not bother to see if this actually leads to problems.

[codecov]

Codecov Report

Attention: Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.

Project coverage is 75.84%. Comparing base (5f8c496) to head (fb8c9b5).
Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
src/GrpAb/Map.jl	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1614      +/-   ##
==========================================
- Coverage   75.95%   75.84%   -0.11%     
==========================================
  Files         362      362              
  Lines      113698   113936     +238     
==========================================
+ Hits        86364    86420      +56     
- Misses      27334    27516     +182     

Files with missing lines	Coverage Δ
src/GrpAb/Elem.jl	96.27% <100.00%> (-0.14%)	⬇️
src/GrpAb/Map.jl	81.64% <71.42%> (+3.52%)	⬆️

... and 53 files with indirect coverage changes