Add new data type Category

[dhlee03]

• Add new data type Category and enum.
• Update test cases accordingly.
  Closes Change data format #16

[codecov]

Codecov Report

Attention: Patch coverage is 79.31034% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 75.25%. Comparing base (9da20be) to head (840f412).

Files	Patch %	Lines
...in/java/seedu/address/model/person/Categories.java	50.00%	1 Missing and 2 partials ⚠️
...main/java/seedu/address/model/person/Category.java	78.57%	2 Missing and 1 partial ⚠️
...in/java/seedu/address/logic/parser/ParserUtil.java	60.00%	1 Missing and 1 partial ⚠️
.../java/seedu/address/storage/JsonAdaptedPerson.java	80.00%	1 Missing and 1 partial ⚠️
...c/main/java/seedu/address/model/person/Person.java	83.33%	0 Missing and 1 partial ⚠️
src/main/java/seedu/address/ui/PersonCard.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #17      +/-   ##
============================================
- Coverage     75.26%   75.25%   -0.01%     
- Complexity      419      429      +10     
============================================
  Files            71       73       +2     
  Lines          1338     1386      +48     
  Branches        126      132       +6     
============================================
+ Hits           1007     1043      +36     
- Misses          301      307       +6     
- Partials         30       36       +6     

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[bachletuankhai]

LGTM