Add classes for Assignment and AddAssignmentCommand

[uyencfi]

• Assignment class implementation
• Assignments are stored under Module, in a UniqueAssignmentList object

[codecov-commenter]

Codecov Report

Merging #82 (d4dc9ff) into master (0fb1d44) will decrease coverage by 4.22%.
The diff coverage is 10.74%.

@@             Coverage Diff              @@
##             master      #82      +/-   ##
============================================
- Coverage     67.40%   63.17%   -4.23%     
- Complexity      454      455       +1     
============================================
  Files            83       90       +7     
  Lines          1500     1613     +113     
  Branches        156      174      +18     
============================================
+ Hits           1011     1019       +8     
- Misses          447      552     +105     
  Partials         42       42              

Impacted Files	Coverage Δ
.../edrecord/logic/commands/AddAssignmentCommand.java	0.00% <0.00%> (ø)
...ava/seedu/edrecord/logic/commands/EditCommand.java	95.94% <ø> (ø)
.../seedu/edrecord/logic/parser/AddCommandParser.java	100.00% <ø> (ø)
...n/java/seedu/edrecord/logic/parser/ParserUtil.java	83.33% <ø> (ø)
src/main/java/seedu/edrecord/model/Model.java	100.00% <ø> (ø)
...va/seedu/edrecord/model/assignment/Assignment.java	0.00% <0.00%> (ø)
...java/seedu/edrecord/model/assignment/MaxScore.java	0.00% <0.00%> (ø)
...ava/seedu/edrecord/model/assignment/Weightage.java	0.00% <0.00%> (ø)
...gnment/exceptions/AssignmentNotFoundException.java	0.00% <0.00%> (ø)
...nment/exceptions/DuplicateAssignmentException.java	0.00% <0.00%> (ø)
... and 10 more

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[marcusc55]

LGTM!

[caipng]

Other than the requested changes, LGTM.

[caipng]

LGTM