Consistently clean output directory

[dschlaep]

We currently remove existing csv-output files at the beginning of each run via SW_CSV_F_INIT(). However, we are not removing existing nc-output files. This is because nc-based SOILWAT2 enables restarts that continue a previously partially completed run.

• for txt-based SOILWAT2: delete all existing files within the output directory at startup. For instance, we could use RemoveFiles() (plus changes if needed) -- and vastly simplify/delete SW_CSV_F_INIT() (though we still need to MkDir() the output directory if not present).

• for nc-based SOILWAT2: delete any lingering ".csv" files but do not delete any ".nc" files (to allow restarts)