Add sort infrastructure

[derekjxtan]

Sortable fields (nric, name, phone, appointmentStartTime, appointmentEndTime) now implements Comparable

Added Comparators for use in model/Sorters

Comparators are untested at this point of time, but can merge if this is blocking. Will be adding tests in due time.

[codecov[bot]]

Codecov Report

Merging #160 (fa97214) into master (d8d1720) will decrease coverage by 1.69%. The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master     #160      +/-   ##
============================================
- Coverage     82.35%   80.66%   -1.69%     
  Complexity      740      740              
============================================
  Files           105      106       +1     
  Lines          2148     2193      +45     
  Branches        214      215       +1     
============================================
  Hits           1769     1769              
- Misses          347      392      +45     
  Partials         32       32              

Files	Coverage Δ
src/main/java/seedu/address/model/person/Name.java	93.33% <0.00%> (-6.67%)	:arrow_down:
src/main/java/seedu/address/model/person/Nric.java	93.75% <0.00%> (-6.25%)	:arrow_down:
...rc/main/java/seedu/address/model/person/Phone.java	93.33% <0.00%> (-6.67%)	:arrow_down:
...edu/address/model/appointment/AppointmentTime.java	81.48% <0.00%> (-18.52%)	:arrow_down:
src/main/java/seedu/address/model/Sorters.java	0.00% <0.00%> (ø)

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