change wapper parameter and inner routine to allow for more felixble index calculation

Currently in wrapper:

        SelectField(
            name="index",
            default="HMD",
            options={"HMD": "HMD", "SPEI": "SPEI", "CSI": "CSI"},
            help=(
                "Select among the following 3 indexes: HMD3 (Heat Magnitude Day), "
                "SPEI6 (Standardized Precipitation Evaporation Index) and CSI "
                "(Combined Stress Index). Please note that CSI will also calculate "
                "HMD3 and SPEI6."
            ),
        ),

we should change to string

        String(
            name="index",
            default="HMD",
            help=(
                """Option to choose index separated via comma \
                HMD,SPEI,CSI CSI would trigger HMD and SPEI.\
                HMD would be default
                """
            ),
        ),

and then change accordingly in [main.py](https://gitlab.dkrz.de/bm1159/plugins4freva/crops/-/blob/master/src/main.py?ref_type=heads)