Update add command to support assets

[yisiox]

Added a field assets as required. GUI is basic, tests are basic (omitted edit related test for now).

Closes #10

[codecov]

Codecov Report

Attention: Patch coverage is 80.23256% with 17 lines in your changes are missing coverage. Please review.

Project coverage is 74.90%. Comparing base (f668a1b) to head (209dd38).

Files	Patch %	Lines
...java/seedu/address/logic/commands/EditCommand.java	76.47%	2 Missing and 2 partials ⚠️
src/main/java/seedu/address/model/asset/Asset.java	81.81%	3 Missing and 1 partial ⚠️
...java/seedu/address/model/person/fields/Assets.java	81.81%	3 Missing and 1 partial ⚠️
src/main/java/seedu/address/ui/PersonCard.java	0.00%	3 Missing ⚠️
...c/main/java/seedu/address/model/person/Person.java	77.77%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #79      +/-   ##
============================================
+ Coverage     74.80%   74.90%   +0.10%     
- Complexity      388      408      +20     
============================================
  Files            66       68       +2     
  Lines          1274     1343      +69     
  Branches        103      110       +7     
============================================
+ Hits            953     1006      +53     
- Misses          292      303      +11     
- Partials         29       34       +5     

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[aureliony]

LGTM