Minor improvements

[RalfG]

Added

• Add + operator support for PSMList

• Add utility functions for m/z-mass conversion in new module psm_utils.utils

• peptidoform: Catch ProFormaError and reraise PeptidoformException with invalid peptidoform in message

  Changed

• Rename optional dependency doc to docs

• Implement "raise from e" when applicable throughout package

[codecov-commenter]

Codecov Report

Patch coverage: 28.20% and project coverage change: +0.01 :tada:

Comparison is base (5c086bc) 42.55% compared to head (5a83bc4) 42.56%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #38 +/- ## ========================================== + Coverage 42.55% 42.56% +0.01% ========================================== Files 19 20 +1 Lines 1537 1553 +16 ========================================== + Hits 654 661 +7 - Misses 883 892 +9 ``` | [Impacted Files](https://app.codecov.io/gh/compomics/psm_utils/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=compomics) | Coverage Δ | | |---|---|---| | [psm\_utils/io/percolator.py](https://app.codecov.io/gh/compomics/psm_utils/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=compomics#diff-cHNtX3V0aWxzL2lvL3BlcmNvbGF0b3IucHk=) | `26.06% <0.00%> (ø)` | | | [psm\_utils/io/tsv.py](https://app.codecov.io/gh/compomics/psm_utils/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=compomics#diff-cHNtX3V0aWxzL2lvL3Rzdi5weQ==) | `37.77% <0.00%> (ø)` | | | [psm\_utils/peptidoform.py](https://app.codecov.io/gh/compomics/psm_utils/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=compomics#diff-cHNtX3V0aWxzL3BlcHRpZG9mb3JtLnB5) | `31.84% <20.00%> (+0.36%)` | :arrow_up: | | [psm\_utils/psm\_list.py](https://app.codecov.io/gh/compomics/psm_utils/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=compomics#diff-cHNtX3V0aWxzL3BzbV9saXN0LnB5) | `59.01% <33.33%> (-0.16%)` | :arrow_down: | | [psm\_utils/utils.py](https://app.codecov.io/gh/compomics/psm_utils/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=compomics#diff-cHNtX3V0aWxzL3V0aWxzLnB5) | `40.00% <40.00%> (ø)` | | | [psm\_utils/io/peptide\_record.py](https://app.codecov.io/gh/compomics/psm_utils/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=compomics#diff-cHNtX3V0aWxzL2lvL3BlcHRpZGVfcmVjb3JkLnB5) | `47.09% <66.66%> (ø)` | |

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