Always use absolute path.

[ReinderVosDeWael]

This PR expands ~ and resolves the root and output paths. This achieves two things:

• Paths relative to home (e.g. ~/repositories/bids2table/) are now accepted.
• All relative paths are resolved to absolute paths. This guards against the current directory changing elsewhere in the code.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (b7b1658) 89.24% compared to head (daed1cc) 89.24%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #12 +/- ## ======================================= Coverage 89.24% 89.24% ======================================= Files 10 10 Lines 437 437 ======================================= Hits 390 390 Misses 47 47 ``` | [Files Changed](https://app.codecov.io/gh/cmi-dair/bids2table/pull/12?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cmi-dair) | Coverage Δ | | |---|---|---| | [bids2table/\_bids2table.py](https://app.codecov.io/gh/cmi-dair/bids2table/pull/12?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cmi-dair#diff-YmlkczJ0YWJsZS9fYmlkczJ0YWJsZS5weQ==) | `90.00% <100.00%> (ø)` | |

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