Update show for GroupAlgebraElem with sparse rep

[lgoettgens]

Let me show the current issue with an example. To reproduce, you need the branch https://github.com/oscar-system/Oscar.jl/tree/lg/character-ring together with a dev'ed Hecke that includes #1685.

julia> R = root_system(:A, 2);

julia> P = weight_lattice(R); # this is basically ZZ^2, but with nice printing

julia> ZP = group_algebra(ZZ, P);

julia> z = zero(ZP)
0

julia> o = one(ZP)
0

julia> z == o # but they print the same
false

julia> 42 * one(ZP) # that should be something like `42 * 0` or `42 * e(0)`
0

julia> a = ZP(gen(P, 1)) + ZP(gen(P, 2))
w_1 + w_2

julia> b = ZP(gen(P, 1) + gen(P, 2))
w_1 + w_2

julia> a == b # but they print the same
false

Thus I propose to instead of just writing the group elem as the basis elem, to do something like e(g) as often done in the literature. With this change enabled, the problematic lines from above change to the following, which IMO is a clear readability and disambiguation improvement.

julia> z = zero(ZP)
0

julia> o = one(ZP)
e(0)

julia> 42 * one(ZP) # that should be something like `42 * 0` or `42 * e(0)`
42*e(0)

julia> a = ZP(gen(P, 1)) + ZP(gen(P, 2))
e(w_1) + e(w_2)

julia> b = ZP(gen(P, 1) + gen(P, 2))
e(w_1 + w_2)

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 75.95%. Comparing base (1b69e3d) to head (b3bec6b).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/AlgAss/Elem.jl	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1688      +/-   ##
==========================================
- Coverage   75.96%   75.95%   -0.01%     
==========================================
  Files         362      362              
  Lines      114325   114325              
==========================================
- Hits        86846    86840       -6     
- Misses      27479    27485       +6     

Files with missing lines	Coverage Δ
src/AlgAss/Elem.jl	88.35% <50.00%> (ø)

... and 28 files with indirect coverage changes

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[fingolfin]

Seems sensible to me