This method takes in a string and instantiates an equip command.
commandStr - The command a user entered to equip a new item.
This method is used to execute an equip command by equipping a weapon or piece of armor. It should follow one of two formats.
A. "equip armor [name of armor piece]"
B. "equip [primary|secondary] [name of weapon]"
If the user is not carrying the following item it returns a string saying so.
Note: If the user already has an item equipped in the listed slot that item will be moved to their inventory.