ENHANCE: CharacteristicSubgroupsLib to CharacteristicSubgroups

[hulpke]

This PR renames CharacteristicSubgroupsLib to CharacteristicSubgroups (as suggested in #975).
As previously discussed in #878, it avoids problems with the declaration in CRISP by removing the ``multiple declarations match'' error, if both declarations are the same.

The actual functionality provided (if CRISP is not loaded) is pathetic and not worth mentioning in the release announcement.

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Copied by @alex-konovalov from commit message: "Also remove multi-declaration warning if both declarations are equal -- this allows the library to duplicate declarations from packages"

[codecov-io]

Current coverage is 49.61% (diff: 95.65%)

Merging #978 into master will decrease coverage by 0.01%

@@             master       #978   diff @@
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  Files           424        424          
  Lines        222957     222959     +2   
  Methods        3430       3430          
  Messages          0          0          
  Branches          0          0          
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- Hits         110661     110629    -32   
- Misses       112296     112330    +34   
  Partials          0          0          

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[olexandr-konovalov]

I've also copied "Also remove multi-declaration warning if both declarations are equal -- this allows the library to duplicate declarations from packages" from the commit message to the comment above - IMHO its important change so it should be more visible there.

[fingolfin]

This looks sensible.

However, these are really two different changes which are bunched together, and one affects all of GAP by modifying how declarations work -- so I'd consider this the by far bigger change. Yet it is not mentioned in the headline of the commit message.

Therefore I'd greatly prefer if the commits could be split into two (I think this it is in general a good idea to keep independent changes in separate commits). This can be done in under two minutes like this:

git checkout branch-with-this-work   # here I assume you have a clean work tree...
git reset HEAD^    # undo the last commit, but leave the changes in work tree
git add -p    # use interactive add to only stage the changes for first commit
git commit    # make first commit
git add -p    # stage remaining changes
git commit

Of course, since the two changes split along file borders nicely, one can also simply explicitly "git add FILES" instead of an interactive add.

[hulpke]

@fingolfin
Just to make sure I understand. You are proposing still one branch, just two commits.

[fingolfin]

@hulpke Yes, exactly as you have done it. Thank you very much!