Implement eq and getitem dunder methods for Peptidoform

[RalfG]

Added

• Peptidoform: Allow comparison between a peptidoform and a peptidoform string
• Peptidoform: Allow direct indexing with square brackets, which indexes or slices parsed_sequence

[codecov[bot]]

Codecov Report

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

Project coverage is 65.19%. Comparing base (8438dbc) to head (34aae96). Report is 3 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #89 +/- ## ========================================== + Coverage 64.73% 65.19% +0.45% ========================================== Files 26 26 Lines 2501 2508 +7 ========================================== + Hits 1619 1635 +16 + Misses 882 873 -9 ``` | [Flag](https://app.codecov.io/gh/compomics/psm_utils/pull/89/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=compomics) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/compomics/psm_utils/pull/89/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=compomics) | `65.19% <100.00%> (+0.45%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=compomics#carryforward-flags-in-the-pull-request-comment) to find out more.

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