Fix bookkeeping overflow

[bryanwee023]

Fixes #199.

Bookkeeping figures overflow due to the lack of checks.

Let's impose a reasonable limit ($999, 999, 999) onto Bookkeeping figures. Revenue or costs above the limit will not be considered. This allows us to perform overflow checks.

[codecov-commenter]

Codecov Report

Merging #202 (08de2f0) into master (8212590) will increase coverage by 0.07%.
The diff coverage is 95.83%.

@@             Coverage Diff              @@
##             master     #202      +/-   ##
============================================
+ Coverage     77.32%   77.39%   +0.07%     
- Complexity      808      809       +1     
============================================
  Files            99       99              
  Lines          2187     2181       -6     
  Branches        311      310       -1     
============================================
- Hits           1691     1688       -3     
  Misses          382      382              
+ Partials        114      111       -3     

Impacted Files	Coverage Δ
...main/java/seedu/address/commons/core/Messages.java	0.00% <ø> (ø)
src/main/java/seedu/address/model/Model.java	100.00% <ø> (ø)
...main/java/seedu/address/model/TransactionList.java	100.00% <ø> (ø)
.../java/seedu/address/model/util/SampleDataUtil.java	12.50% <0.00%> (ø)
.../java/seedu/address/logic/commands/AddCommand.java	83.87% <100.00%> (ø)
...in/java/seedu/address/logic/parser/ParserUtil.java	89.58% <100.00%> (ø)
src/main/java/seedu/address/model/BookKeeping.java	96.77% <100.00%> (+11.92%)	⬆️
...rc/main/java/seedu/address/model/ModelManager.java	91.36% <100.00%> (-0.13%)	⬇️
...u/address/storage/JsonSerializableBookKeeping.java	100.00% <100.00%> (ø)
src/main/java/seedu/address/model/UserPrefs.java	94.87% <0.00%> (-2.57%)	⬇️
... and 1 more

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