Modify storage to handle expense and income

[yongping827]

Partially addresses #59.

JSON data files now require "transactions", "expenses" and "incomes" fields. "transactions" will be removed in a future PR. Loading and saving of current data reflects this structure.

Adding expenses and incomes does not work yet, since they are still added to transaction list, instead of expense and income lists.

[wltan]

Pitest report: https://ay2021s1-cs2103t-w16-3.github.io/reports/pitest/202010081253/

Note that the mutation coverage for ExpenseList and IncomeList is not very good, as the tests are based on the original AB3. It would be good if we could take a brief look to see if it can be improved, but not a hard requirement at the moment.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@3e647c1). Click here to learn what that means.
The diff coverage is 88.50%.

@@            Coverage Diff            @@
##             master      #76   +/-   ##
=========================================
  Coverage          ?   70.10%           
  Complexity        ?      450           
=========================================
  Files             ?       82           
  Lines             ?     1495           
  Branches          ?      141           
=========================================
  Hits              ?     1048           
  Misses            ?      410           
  Partials          ?       37           

Impacted Files	Coverage Δ	Complexity Δ
...s2103_w16_3/finesse/model/util/SampleDataUtil.java	20.00% <ø> (ø)	1.00 <0.00> (?)
...1s1_cs2103_w16_3/finesse/model/FinanceTracker.java	70.83% <55.55%> (ø)	17.00 <6.00> (?)
...3_w16_3/finesse/model/transaction/ExpenseList.java	85.71% <85.71%> (ø)	10.00 <10.00> (?)
...03_w16_3/finesse/model/transaction/IncomeList.java	85.71% <85.71%> (ø)	10.00 <10.00> (?)
...2103_w16_3/finesse/storage/JsonAdaptedExpense.java	100.00% <100.00%> (ø)	9.00 <9.00> (?)
...s2103_w16_3/finesse/storage/JsonAdaptedIncome.java	100.00% <100.00%> (ø)	9.00 <9.00> (?)
...inesse/storage/JsonSerializableFinanceTracker.java	100.00% <100.00%> (ø)	5.00 <0.00> (?)

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[wltan]

LGTM for now. As noted in the PR description more work is needed before #59 is resolved.

[zhaojj2209]

LGTM!
Next steps:

1. Remodel Model and Logic such that Expense is added to ExpenseList and Income is added to IncomeList
2. Make the correct list display in the correct tab in the UI
3. Remove TransactionList and make Transaction an abstract class