Fix xtandem `run` value parsing; fix renaming negative mass modifications

[ArthurDeclercq]

Added

• Tests: Added tests for _format_number_as_string function
• Tests: Added more test cases for peptidoform.rename_modifications for mass modifications
• io.xtandem: To parse run value, fall back to PSM file name if run name cannot be parsed from label field

Fixed

• peptidoform.rename_modifications: Fixed mapping of negative mass modifications
• io.xtandem: Fixed regular expression to parse run value fom XML label field

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (37163f1) 61.72% compared to head (13b367b) 61.70%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #62 +/- ## ========================================== - Coverage 61.72% 61.70% -0.03% ========================================== Files 23 23 Lines 2336 2340 +4 ========================================== + Hits 1442 1444 +2 - Misses 894 896 +2 ``` | [Files](https://app.codecov.io/gh/compomics/psm_utils/pull/62?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=compomics) | Coverage Δ | | |---|---|---| | [psm\_utils/peptidoform.py](https://app.codecov.io/gh/compomics/psm_utils/pull/62?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=compomics#diff-cHNtX3V0aWxzL3BlcHRpZG9mb3JtLnB5) | `38.63% <0.00%> (ø)` | | | [psm\_utils/io/xtandem.py](https://app.codecov.io/gh/compomics/psm_utils/pull/62?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=compomics#diff-cHNtX3V0aWxzL2lvL3h0YW5kZW0ucHk=) | `69.49% <40.00%> (-1.42%)` | :arrow_down: |

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