[MNT] Refactor AptaNetPSeAAC as thin subclass of PSeAAC (#541)

#### Reference Issues/PRs
Fixes #537

#### What does this implement/fix? Explain your changes.

`AptaNetPSeAAC` in `pyaptamer/pseaac/_aptanet_pseaac.py` was a verbatim
copy of `PSeAAC`, duplicating `_normalized_aa`, `_avg_theta_val`, and
`transform` with no additional logic. This created a maintenance risk
where any bug fixed in `PSeAAC` would have to be manually mirrored to
`AptaNetPSeAAC`.

This PR refactors `AptaNetPSeAAC` into a thin subclass of `PSeAAC`,
removing all duplicated methods and delegating to `super().__init__()`
with the fixed 21-property, 7-group AptaNet configuration. The docstring
example was also corrected — it previously imported and instantiated
`PSeAAC` instead of `AptaNetPSeAAC`.

#### What should a reviewer concentrate their feedback on?

- The `super().__init__()` call in `AptaNetPSeAAC.__init__` and whether
the passed parameters (`prop_indices=None`, `group_props=3`) correctly
reproduce the original AptaNet configuration
- The updated docstring example for correctness

#### Did you add any tests for the change?

No new tests were added. A manual check confirms the output which was
identical to the original implementation.

#### Any other comments?

This change has been discussed with @satvshr on Discord who confirmed
the refactor is welcome.

#### PR checklist
- [x] The PR title starts with either [ENH], [MNT], [DOC], or [BUG].
[BUG] - bugfix, [MNT] - CI, test framework, [ENH] - adding or improving
code, [DOC] - writing or improving documentation or docstrings.
- [ ] Added/modified tests
- [x] Used pre-commit hooks when committing to ensure that code is
compliant with hooks. Install hooks with `pre-commit install`.
To run hooks independent of commit, execute `pre-commit run --all-files`

Author: SanchitKS12