Add in interpolation feature for kwargs parsing

[mfherbst]

CC @WuyihanVictor

The way to use this is to add to the scf "$kwargs":

            "nbandsalg": {
                 "$symbol": "AdaptiveBands",
                 "$args": ["$model"],
                 "$kwargs": {
                     "n_bands_converge": 8
                 }
            },