[Feature Request] pt: fix get_sel, get_type, etc. in linear_atomic_models

[njzjz]

[wanghan-iapcm]

I find the behavior of get_sel, get_type of linear atomic model is the same as that of the hybrid descriptor.

but the linear atomic model assumes that all the sub-models are of mixed type. This restriction should be checked at constructor and an error is expected to raise when the restriction is not fulfilled.