add fixed=TRUE pattern mode to chooseFromList

[orichters]

• chooseFromList always had a mode for using regular expressions
• that is extremely annoying if you want to select AR6 / remind2 variables that contain several |
• add fixed=TRUE mode so that user can decide whether that pattern should not be interpreted as regular expression

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.27 :tada:

Comparison is base (4dfce43) 34.05% compared to head (eb4dce1) 34.33%.

:exclamation: Current head eb4dce1 differs from pull request most recent head e5d4b8c. Consider uploading reports for the commit e5d4b8c to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #64 +/- ## ========================================== + Coverage 34.05% 34.33% +0.27% ========================================== Files 51 51 Lines 1665 1672 +7 ========================================== + Hits 567 574 +7 Misses 1098 1098 ``` | [Impacted Files](https://app.codecov.io/gh/pik-piam/gms/pull/64?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pik-piam) | Coverage Δ | | |---|---|---| | [R/chooseFromList.R](https://app.codecov.io/gh/pik-piam/gms/pull/64?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pik-piam#diff-Ui9jaG9vc2VGcm9tTGlzdC5S) | `88.46% <100.00%> (+1.13%)` | :arrow_up: |

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