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
         }