Elastic: get started and input examples

Here we calculate the mechanical properties which include elastic constants (C11 to C66), bulk modulus Bv, shear modulus Gv, Youngs modulus Ev, and Poission ratio Uv of a certain crystal structure.

An example of the input file for Elastic by deepmd:

{
	"structures":	["confs/mp-*","confs/std-*","confs/test-*"],
	"interaction": {
		"type":		"deepmd",
                "model":        "frozen_model.pb",
		"type_map":	{"Al": 0, "Mg": 1}
	},
	"properties": [
            {
                "type":         "elastic",
                "norm_deform":	2e-2,
	       "shear_deform":  5e-2
	    }
        ]
}

Here the default values of norm_deform and shear_deform are 2e-3 and 5e-3, respectively. A list of norm_strains and shear_strains would be generated as below:

[-norm_def, -0.5 * norm_def, 0.5 * norm_def, norm_def]
[-shear_def, -0.5 * shear_def, 0.5 * shear_def, shear_def]