Add default unit and variable name mapping for NWM temperature

[samlamont]

can add "T2D" to the variable mapper in teehr/fetching/const.py:

    VARIABLE_NAME: {
        "streamflow": "streamflow_hourly_inst",
        "RAINRATE": "rainfall_hourly_rate",
        "T2D": "temperature_hourly_mean"
    },

and add "temperature_hourly_mean" to the template variable_name table and "K" to the units table