Fix bug in FixedPoints for a partial perm coll

[james-d-mitchell]

Please make sure that this pull request:

• is submitted to the correct branch (the stable branch is only for bugfixes)
• contains an accurate description of changes for the release notes below
• provides new tests or relies on existing ones
• correctly refers to other issues and related pull requests

Tick all what applies to this pull request

• Adds new features
• Improves and extends functionality
• Fixes bugs that could lead to crashes
• Fixes bugs that could lead to incorrect results
• Fixes bugs that could lead to break loops

Write below the description of changes (for the release notes)

Previously NrFixedPoints and FixedPointsOfPartialPerm for a partial perm and a partial perm semigroup, respectively, returned the moved points instead. Also some tests are added.

[codecov-io]

Current coverage is 49.56% (diff: 100%)

Merging #1034 into master will increase coverage by 0.03%

@@             master      #1034   diff @@
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  Files           424        424          
  Lines        223274     223274          
  Methods        3430       3430          
  Messages          0          0          
  Branches          0          0          
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+ Hits         110593     110674    +81   
+ Misses       112681     112600    -81   
  Partials          0          0          

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