change the handling of the `init` argument in `map_word`

[ThomasBreuer]

resolves #3885

• use init whenever it is different from nothing (this differs from the documented behaviour for the Julia functions sum and prod)

• extend the map_word method for PcGroupElem to SubPcGroupElem

• add a special map_word method where the images are FinGenAbGroupElems, where evaluating means to add images instead of multiplying them

• extend the tests

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.87%. Comparing base (09c52c1) to head (3464c5b). Report is 4 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3892 +/- ## ========================================== + Coverage 83.81% 83.87% +0.06% ========================================== Files 582 589 +7 Lines 80343 80897 +554 ========================================== + Hits 67338 67851 +513 - Misses 13005 13046 +41 ``` | [Files](https://app.codecov.io/gh/oscar-system/Oscar.jl/pull/3892?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=oscar-system) | Coverage Δ | | |---|---|---| | [src/Groups/GAPGroups.jl](https://app.codecov.io/gh/oscar-system/Oscar.jl/pull/3892?src=pr&el=tree&filepath=src%2FGroups%2FGAPGroups.jl&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=oscar-system#diff-c3JjL0dyb3Vwcy9HQVBHcm91cHMuamw=) | `94.13% <100.00%> (+0.35%)` | :arrow_up: | ... and [10 files with indirect coverage changes](https://app.codecov.io/gh/oscar-system/Oscar.jl/pull/3892/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=oscar-system)