Fix One/Zero/MultiplicativeZero for partial perms

[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)

There were several issues here:

1. some declarations were missing

2. for a partial perm semigroup without generators the default method or
   a One of a partial perm collection was used. This always returned an
   answer, when it shouldn't have it if the One was not in the semigroup.

3. partial perms and semigroups of partial perms had methods for Zero,
   which according to the manual "return the additive neutral element of
   the additive element". Since partial perms are not additive these
   methods were inappropriate. As such they are removed in this PR and
   methods for MultiplicativeZero are introduced instead. Partial perms
   should have belonged to the category IsMultiplicativeElementWithZero,
   and so I added this to the definition of IsPartialPerm in pperm.g.
   This meant moving the declaration of IsMultiplicativeElementWithZero
   from mgmadj.gd to arith.gd so that it is available when pperm.g is
   read (from read1.g). There was no default method for the attribute
   MultiplicativeZero for an object in IsMultipicativeElementWithZero
   so I added one of these too, it simply calls MultiplicativeZeroOp.

This commit fixes these issues.

[codecov-io]

Current coverage is 49.60% (diff: 100%)

Merging #1040 into master will decrease coverage by <.01%

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