Vacancy: get started and input examples

Vacancy calculates the energy difference when removing an atom from the crystal structure. We only need to give the information of supercell to help calculate the vacancy energy and the default value of supercell is [1, 1, 1].

An example of the input file for Elastic by deepmd:

{
	"structures":	"confs/mp-*",
	"interaction": {
		"type":		"deepmd",
                "model":        "frozen_model.pb",
		"type_map":	{"Al": 0, "Mg": 1}
	},
	"properties": [
            {
                "type":         "vacancy",
                "supercell":	[1, 1, 1]
	    }
        ]
}