maven learner lab from ghassan nasr#17

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ghassannasr wants to merge 8 commits intocurriculeon:masterfrom

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Commits on Jul 21, 2020

Commits on Jul 22, 2020

completed 6.1 and 6.0
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Commits on Jul 24, 2020

finished part 7, singleton class, consulted several sources and feel I have a better grip on the design pattern
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completed part 8, added an Instructor constructor that gives an Instructor and id and a name by calling the Person constructor, since in the the singleton instructors exist as a List of Persons
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Commits on Jul 29, 2020

made important changes to Students and Instructors singletons (parts 7 and 8), and implemented and tested Classroom (part 9)
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Commits on Aug 1, 2020

completed part 10
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