Update Model and Logic to support View commands

[yanghengtang]

1. The following attributes, together with its getter and setter, have been added to Model interface
   • selectedAppointment
   • selectedDoctor
   • selectedPatient
2. Logic interface will also have getters for all the attributes in Model
3. CommandResult will have the following attributes, together with getters, two new constructors
   • showAppointment
   • showDoctor
   • showPatient

All these will be used in the implementation of the View command.

[codecov[bot]]

Codecov Report

Merging #130 (79634e1) into master (6796bcc) will increase coverage by 0.23%. The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #130      +/-   ##
============================================
+ Coverage     81.99%   82.23%   +0.23%     
- Complexity      712      729      +17     
============================================
  Files           105      105              
  Lines          2088     2116      +28     
  Branches        214      214              
============================================
+ Hits           1712     1740      +28     
  Misses          344      344              
  Partials         32       32              

Files	Coverage Δ
...rc/main/java/seedu/address/logic/LogicManager.java	100.00% <100.00%> (ø)
...va/seedu/address/logic/commands/CommandResult.java	100.00% <100.00%> (ø)
src/main/java/seedu/address/model/Model.java	100.00% <ø> (ø)
...rc/main/java/seedu/address/model/ModelManager.java	95.87% <100.00%> (+0.58%)	:arrow_up:

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