Add calculations of budget and savings

[zhaojj2209]

Resolves #170.
Resolves #171.

Changes:

• ObservableAmount is no longer abstract to avoid duplication of similar classes
• Added calculations of monthly expenses, monthly incomes and monthly savings for the past 3 months in MonthlyBudget; these values are not stored in the finance tracker but are calculated whenever the amounts in the finance tracker's transactions changes.
• Added a boolean to CommandResult to track when the budget information in MonthlyBudget needs to be recalculated.
• Added an overloaded constructor to assertCommandSuccess to pass tests; likely a temporary fix, will try to think of something better down the line.
• All calculations of the current month's budget/savings will be displayed in the SavingsGoalPanel for easy reference for now.

[codecov-io]

Codecov Report

Merging #172 into master will decrease coverage by 0.52%.
The diff coverage is 72.02%.

@@             Coverage Diff              @@
##             master     #172      +/-   ##
============================================
- Coverage     76.03%   75.51%   -0.53%     
- Complexity      771      787      +16     
============================================
  Files           131      129       -2     
  Lines          2270     2381     +111     
  Branches        243      252       +9     
============================================
+ Hits           1726     1798      +72     
- Misses          447      488      +41     
+ Partials         97       95       -2     

Impacted Files	Coverage Δ	Complexity Δ
...021s1_cs2103_w16_3/finesse/logic/LogicManager.java	81.48% <0.00%> (-6.52%)	9.00 <0.00> (ø)
...ava/ay2021s1_cs2103_w16_3/finesse/model/Model.java	100.00% <ø> (ø)	5.00 <0.00> (ø)
...a/ay2021s1_cs2103_w16_3/finesse/ui/MainWindow.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...21s1_cs2103_w16_3/finesse/ui/SavingsGoalPanel.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...cs2103_w16_3/finesse/ui/tabs/AnalyticsTabPane.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...2103_w16_3/finesse/logic/commands/EditCommand.java	94.11% <75.00%> (-1.27%)	14.00 <1.00> (+1.00)	⬇️
...3_w16_3/finesse/model/budget/ObservableAmount.java	53.84% <75.00%> (-46.16%)	4.00 <1.00> (-1.00)
...2103_w16_3/finesse/model/budget/MonthlyBudget.java	85.71% <86.48%> (+2.38%)	19.00 <14.00> (+13.00)
...16_3/finesse/logic/commands/AddExpenseCommand.java	100.00% <100.00%> (ø)	7.00 <0.00> (ø)
...w16_3/finesse/logic/commands/AddIncomeCommand.java	100.00% <100.00%> (ø)	7.00 <0.00> (ø)
... and 20 more

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[yongping827]

Addition of expenses does not affect the remaining budget.

[yongping827]

LGTM

[ianyong]

LGTM!