Repair broken abstraction - Amount

[wltan]

Fixes #279

Changes made:

• Introduced a new class CalculatedAmount. Unlike Amount, it can contain negative values inside without data integrity issues. It also supports addition and subtraction, which was previously done (incorrectly) outside using the underlying BigDecimal types.
  • CalculatedAmount is deliberately not a subtype of Amount, due to them representing different things, despite sharing a similar implementation.
• Removed from Amount methods that are no longer necessary, such as getValue and isNonNegative. These methods have been moved to CalculatedAmount instead.
• Removed from Amount the of method and its associated constructor. It is no longer needed.
• Changed the type of the following properties in MonthlyBudget
  • monthlyExpenseLimit: ObservableAmount -> ObjectProperty<Amount>
  • monthlySavingsGoal: ObservableAmount -> ObjectProperty<Amount>
  • remainingBudget: ObservableAmount -> ObjectProperty<CalculatedAmount>
  • currentSavings: ObservableAmount -> ObjectProperty<CalculatedAmount>
  • Expense limit and savings goals are non-negative and are created from commands, so they have been parsed as Amounts and are maintained as Amounts.
  • Budget and savings are derived values, so they are maintained as CalculatedAmounts.
• Removed the ObservableAmount class.
• Changed the budget calculation to use CalculatedAmount instead of BigDecimal.

Pitest: https://ay2021s1-cs2103t-w16-3.github.io/reports/pitest/202011060226

• The coverage has remained unchanged compared to master branch.