Add new field in the model of POS module

To add new field in POS modules necessary in models.js override PosModel in the parent models which we take from “point_of_sale.models”. For example:

var models = require('point_of_sale.models');
var _super_posmodel = models.PosModel.prototype;

models.PosModel = models.PosModel.extend({
    initialize: function (session, attributes) {
        // New code
        var partner_model = _.find(this.models, function(model){
            return model.model === 'product.product';
        });
        partner_model.fields.push('qty_available');

        // Inheritance
        return _super_posmodel.initialize.call(this, session, attributes);
    },
});