checks if model file exists, requires it and returns it.
@param filePath
@param cb
function requireModel(filePath, cb) {
            //check if file exist. if not return error
            fs.exists(filePath, function (exists) {
                if (!exists) {
                    return cb(new Error("Requested Model Not Found!"));
                }

                libProduct.model = require(filePath);
                return cb(null, libProduct);
            });
        }