System.from does not suppot the mixed_type data

[Manyi-Yang]

The model of lmp does not support "fmt" : "deepmd/npy/mixed",

 "convergence": {
            "type":                    "adaptive-lower",
           "conv_tolerance":            0.00,
           "numb_candi_f":             10,
           "_rate_candi_f":              0,
           "level_f_hi":                0.5,
           "n_checked_steps":           8,
           "_command":      "all"
        },
        "max_numb_iter" :       1,
        "fatal_at_max" :        false,
        "output_nopbc":         true,
        "configuration_prefix": null,
        "configurations":       [
            {
                "type" : "file",
                "files" : ["md.data/35"],
                "fmt" : "deepmd/npy/mixed",
                "remove_pbc" : true
            }