feat: add regular_character

[thofma]

CC: @joschmitt

[joschmitt]

This would resolve #3390 assuming we export class_function, right?

[thofma]

Exporting class_function would be more complicated, so let's keep this PR here focused on adding regular_character.

[codecov]

Codecov Report

Merging #3393 (a61221d) into master (a73b844) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3393   +/-   ##
=======================================
  Coverage   82.10%   82.10%           
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  Files         557      557           
  Lines       73971    73975    +4     
=======================================
+ Hits        60734    60738    +4     
  Misses      13237    13237           

Files	Coverage Δ
src/Groups/group_characters.jl	95.34% <100.00%> (+0.02%)	⬆️

[ThomasBreuer]

Actually regular_character can assume that the class of the identity element is the first one, thus the function need not do computations with the group.
(I should state this fact in the docstring for conjugacy_classes for character tables.)

What about providing regular_character(tbl::GAPGroupCharacterTable),
and defining regular_character(G::GAPGroup) = regular_character(character_table(G))?

[fingolfin]

I think what @ThomasBreuer suggests would be the way to go. Of course we can also do that later if you need this in immediately.

[thofma]

Adjusted following your comments. Please have a look again @ThomasBreuer

[ThomasBreuer]

Looks good. Thanks. Just one minor comment on a docstring.