`clean_wqdata()` is incorrectly changing detected values to non-detects

This was previously reported but the error was not fixed by the standardize_mdl_units() rems function https://github.com/bcgov/wqbc/issues/158

I think this is partly happening because the columns change names in standardize_wqdata() and are not recognized in the rems function.

I think the best way to fix might be to:
- add MDL_UNIT to list of cols kept in standardize_wqdata()
- add standardize_mdl_units to standardize_wqdata()
- fix clean_wqdata() function so it does not modify the ResultLetter column

...but open to other thoughts!

Also an issue identified in shinyrems https://github.com/bcgov/shinyrems/issues/123

Start by comparing the column names and retained fields across standardize_wqdata(), standardize_mdl_units(), and clean_wqdata(), with issue 158 as context. Check how the rems function handles MDL_UNIT and ResultLetter, then verify that detected values remain detected after cleaning. The related shinyrems issue 123 may provide additional context.

Written by the indexing model from the issue text.