Fix model constructors

[derekjxtan]

Overloaded constructors for Person, Patient, Doctor and Appointment to allow initialization with and without remarks and tags.

Note to those that will be doing tags, general way to work with tags will be:

1. On add, use the current constructor without remark and tags.
2. On add/remove remarks
   1. call getTags() to get the set of tags
   2. call tags.remove() or tags.add() to add/remove tags.
   3. create a new Instance with the modified tag set through the new constructor.

Data class is already created to be immutable so dont need to worry about causing problems to the existing models.

closes #132

[codecov[bot]]

Codecov Report

Merging #129 (074c682) into master (6796bcc) will increase coverage by 0.15%. The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #129      +/-   ##
============================================
+ Coverage     81.99%   82.14%   +0.15%     
- Complexity      712      716       +4     
============================================
  Files           105      105              
  Lines          2088     2106      +18     
  Branches        214      214              
============================================
+ Hits           1712     1730      +18     
  Misses          344      344              
  Partials         32       32              

Files	Coverage Δ
...a/seedu/address/model/appointment/Appointment.java	100.00% <100.00%> (ø)
...c/main/java/seedu/address/model/person/Person.java	100.00% <100.00%> (ø)
...java/seedu/address/model/person/doctor/Doctor.java	90.00% <100.00%> (+1.11%)	:arrow_up:
...va/seedu/address/model/person/patient/Patient.java	100.00% <100.00%> (ø)

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