Change sorting implementation of appointments

[ktaekwon000]

No description provided.

[codecov]

Codecov Report

Merging #147 into implement-appointment-class will not change coverage.
The diff coverage is 50.00%.

@@                      Coverage Diff                       @@
##             implement-appointment-class     #147   +/-   ##
==============================================================
  Coverage                          62.68%   62.68%           
  Complexity                           594      594           
==============================================================
  Files                                102      102           
  Lines                               2136     2136           
  Branches                             255      255           
==============================================================
  Hits                                1339     1339           
  Misses                               709      709           
  Partials                              88       88           

Impacted Files	Coverage Δ	Complexity Δ
...in/java/seedu/address/ui/AppointmentListPanel.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...dress/model/appointment/UniqueAppointmentList.java	21.95% <100.00%> (ø)	4.00 <1.00> (ø)

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