I started to add logic to handle the increase health capacity powerup.
I added logic to increase the players health capacity from 10 to 20 iin the form of a powerup.

I still need to do the following:

1) Add respective powerups to increase health capacity from 20 to 40.
2) Add respective powerups to increase sheild powerups from 10 to 20 and from 20 to 40.
3) Add logic so that each powerup only spawns after a certain score has been reached.
4) Add logic so that once the player is equipped with an increase in capacity for either health or shields, it does not appear again.
I added powerups to increase the players health and shield capacity. Now, when the player begins the game, the player's ship has 10 units of health and shields. I added two power-up to increase both the ship's health and shield capacity from 10 to 20 and from 20 to 40.

When the power-ups appear is dependent on the player's score. After a certain score both power-ups to increase the health and shield capacity from 10 to 20 will begin to appear and the same goes for the power-ups to increase the ship's capacity from 20 to 40.

I also made some other power-ups appear when certain conditions are met. For instance only full health and full shield power-ups will appear if the player's health or shields are less than full. The firepower power-ups will not appear if the player has the maximum fire power.
I forgot to add the file for the class PowerupIncreasedShieldCapacity in the last commit.
I refactored the attribute classes. I added an initialize method to the IAttribute interace and eliminated the unused constructors from the different attribute classes. I also cleaned up the PlayerAttributes class by calling each attributes initialize method from within the PlayerAttribute class rather than calling the setValue and setLimit methods individually.

 also eliminted the AttributeBase class since it was not being used. I originally created this abstract class that implemented the IAttribute interface (as abstract methods) and let the specific player attributes (e.g., AttributeHealth) extend this base class. However, since there was no common logic in the AttributeBase class that was common to all attributes, I decided that this base class was no longer needed.