Replace loan value types with BigDecimals

[kjw142857]

Fixes #121.

• Replaces all representation of loan values as int/float with BigDecimal, which prevents string conversion errors.
• Updated the JavaDoc documentation.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 36 lines in your changes are missing coverage. Please review.

Project coverage is 47.04%. Comparing base (8b0e1d6) to head (91cb2c5).

Files	Patch %	Lines
...ain/java/seedu/address/model/person/Analytics.java	0.00%	15 Missing ⚠️
...ava/seedu/address/model/person/UniqueLoanList.java	0.00%	4 Missing ⚠️
src/main/java/seedu/address/ui/AnalyticsPanel.java	0.00%	4 Missing ⚠️
src/main/java/seedu/address/model/person/Loan.java	0.00%	3 Missing ⚠️
.../seedu/address/logic/commands/EditLoanCommand.java	0.00%	2 Missing ⚠️
...in/java/seedu/address/logic/parser/ParserUtil.java	0.00%	2 Missing ⚠️
...a/seedu/address/model/analytics/DashboardData.java	0.00%	2 Missing ⚠️
.../java/seedu/address/model/util/SampleDataUtil.java	0.00%	2 Missing ⚠️
.../seedu/address/logic/commands/LinkLoanCommand.java	0.00%	1 Missing ⚠️
...rc/main/java/seedu/address/model/ModelManager.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #123      +/-   ##
============================================
- Coverage     47.06%   47.04%   -0.03%     
  Complexity      433      433              
============================================
  Files            99       99              
  Lines          2267     2268       +1     
  Branches        227      227              
============================================
  Hits           1067     1067              
- Misses         1155     1156       +1     
  Partials         45       45              

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[marcus-ny]

Changes lgtm! 👍