added Assignment, AssignmentList, Submission classes

[TamKengHong]

No description provided.

[codecov-commenter]

Codecov Report

Merging #66 (37da30e) into master (af5471a) will decrease coverage by 2.75%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master      #66      +/-   ##
============================================
- Coverage     71.90%   69.15%   -2.75%     
  Complexity      399      399              
============================================
  Files            71       76       +5     
  Lines          1235     1284      +49     
  Branches        125      127       +2     
============================================
  Hits            888      888              
- Misses          314      363      +49     
  Partials         33       33              

Impacted Files	Coverage Δ
...main/java/seedu/address/assignment/Assignment.java	0.00% <0.00%> (ø)
.../java/seedu/address/assignment/AssignmentList.java	0.00% <0.00%> (ø)
...rc/main/java/seedu/address/assignment/Student.java	0.00% <0.00%> (ø)
...ain/java/seedu/address/assignment/StudentList.java	0.00% <0.00%> (ø)
...main/java/seedu/address/assignment/Submission.java	0.00% <0.00%> (ø)

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[bojie3]

LGTM