coverage_daggers: Dagger or double dagger according to coverage level
Description
Transform coverage columns in Rrepest to dagger and double dagger according
to the coverage (cvge) column. Default levels are 75 (dagger) and 50 (double dagger).
If coverage is above both levels, no symbol is produced (empty). Used with the coverage o
option in Rrepest set to TRUE to produce columns with coverage percentages.