Problem:
Write programs to implement a Real Estate Presentation Center that shows no more than 50 houses.

Your Task:
1. Create a class called "House" that includes
a) Private attributes: "houseID","price","size","address"
b) A constructor that sets all attributes
c) A method called "changePrice"that changes the price of the house.
d) A method called "pricePerSqft"that returns the house price for each square foot.
e) A method called "toString" that shows the houseID, price, size and the address in a
single line.

2. Create a class called "TownHouse"that is a subclass of "House". Your class should include the following.
a) An additional private attribute: MonthlyMaintenanceFee
b) A constructor that sets all attributes.
c) A method called "toString" that returns a String that shows the houseID, price,
size, address and MonthlyMaintenanceFee in a single line.

3. Create a class called "RealEstatePresentationCenter" that can be used to store, change and show the housing information. Your class should include the following.
a) Private attributes"listingHouses"and "houseCount"
b) A contractor
c) A method called "addHouse"that adds a house to "listingHouses".
d) A method called "getLowestPricePerSqft" that returns a String that represents
the information of the house with the lowest price for each square foot in the
presentation center.
e) A method called "averageHousePrice" that returns the average house prices.
f) A method called "showAllHouses" that takes returns the information of all
houses in the presentation center.

4. Create a class called "RealEstateTest.java"that tests House.java, TownHouse.java, and RealEstatePresentationCenter.java. You should add several houses and townhouses to the listinghouses, change house prices and test the other methods you implemented.