Add more tests and check code quality

[butteredyakiimo]

• created a method to check if a person is an available fosterer and whether a person is a current fosterer in person class
• made a modification to the statsavail self invocation diagram

Note: codecov is failing because i modified the person class. What i added are already covered!!

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (b426619) 63.23% compared to head (69dbd86) 63.40%.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #145      +/-   ##
============================================
+ Coverage     63.23%   63.40%   +0.17%     
- Complexity      730      733       +3     
============================================
  Files           104      104              
  Lines          2927     2927              
  Branches        330      329       -1     
============================================
+ Hits           1851     1856       +5     
+ Misses          997      993       -4     
+ Partials         79       78       -1     

Files	Coverage Δ
...eedu/address/logic/commands/StatsAvailCommand.java	100.00% <100.00%> (ø)
...ava/seedu/address/logic/commands/StatsCommand.java	20.00% <ø> (ø)
...du/address/logic/commands/StatsCurrentCommand.java	100.00% <100.00%> (+2.38%)	⬆️
...du/address/logic/commands/StatsHousingCommand.java	100.00% <100.00%> (ø)
...java/seedu/address/commons/core/index/Indices.java	94.00% <0.00%> (ø)
...c/main/java/seedu/address/model/person/Person.java	95.87% <66.66%> (+4.66%)	⬆️

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[nananakx-x]

lgtm, I will look at the tests in detail when I am adding on :P