write a full class definition for a class named gastank, and containing the following members: a data member named amount of type double. a constructor that accepts no parameters. the constructor initializes the data member amount to 0. a function named addgas that accepts a parameter of type double. the value of the amount instance variable is increased by the value of the parameter. a function named usegas that accepts a parameter of type double. the value of the amount data member is decreased by the value of the parameter. a function named getgaslevel that accepts no parameters. getgaslevel returns the value of the amount data member.