Add limit and tests for Amount

[jordanyoong]

Limit Amount to have a value of not more than 1000000.00

[codecov-io]

Codecov Report

Merging #148 into master will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #148      +/-   ##
============================================
+ Coverage     55.02%   55.09%   +0.07%     
- Complexity      426      428       +2     
============================================
  Files            96       96              
  Lines          1772     1775       +3     
  Branches        194      195       +1     
============================================
+ Hits            975      978       +3     
  Misses          722      722              
  Partials         75       75              

Impacted Files	Coverage Δ	Complexity Δ
...java/seedu/address/model/account/entry/Amount.java	71.42% <100.00%> (+7.79%)	6.00 <3.00> (+2.00)

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[Ziyang-98]

LGTM