Fix bug #215

[sc-arecrow]

Fix bug #215

Summary

A student can be edited to have the same NUSNET ID of another existing
student in the list.

A student is identified only by their NUSNET ID, all other fields are
not identity fields.

Let's fix the isSameStudent method to reflect this design.
Closes #215

Type of change

<Select all that apply, in the form [x]>

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[codecov]

Codecov Report

Merging #217 (a8a6ca0) into master (a53e443) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #217      +/-   ##
============================================
- Coverage     79.59%   79.57%   -0.03%     
+ Complexity      692      688       -4     
============================================
  Files            97       97              
  Lines          1985     1983       -2     
  Branches        243      240       -3     
============================================
- Hits           1580     1578       -2     
  Misses          298      298              
  Partials        107      107              

Impacted Files	Coverage Δ	Complexity Δ
...n/java/seedu/taskmaster/model/student/Student.java	97.56% <100.00%> (-0.12%)	18.00 <0.00> (-4.00)
...eedu/taskmaster/model/session/SessionDateTime.java	92.30% <0.00%> (ø)	9.00% <0.00%> (ø%)

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[jaya-r3]

LGTM