Add undo command

[Ziyang-98]

I add new method to Account so that GetTotalCommand and GetProfitCommand's code quality is improved. I added assertions to all the Commands as well. Lastly, I implemented the UndoCommand. I only allow UndoCommand for editing, deleting and adding of entries. Also, if the user switches account, the user will not be able to undo the previous commands even if the user switches back.

[lettuceman4]

lgtm

[codecov-io]

Codecov Report

Merging #149 into master will decrease coverage by 0.26%.
The diff coverage is 27.53%.

@@             Coverage Diff              @@
##             master     #149      +/-   ##
============================================
- Coverage     52.80%   52.53%   -0.27%     
- Complexity      428      431       +3     
============================================
  Files            97       98       +1     
  Lines          1854     1892      +38     
  Branches        202      204       +2     
============================================
+ Hits            979      994      +15     
- Misses          800      819      +19     
- Partials         75       79       +4     

Impacted Files	Coverage Δ	Complexity Δ
...java/seedu/address/logic/commands/EditCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...java/seedu/address/logic/commands/ExitCommand.java	100.00% <ø> (ø)	2.00 <0.00> (ø)
...seedu/address/logic/commands/GetProfitCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
.../seedu/address/logic/commands/GetTotalCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...java/seedu/address/logic/commands/ListCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...u/address/logic/commands/SwitchAccountCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...java/seedu/address/logic/commands/UndoCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...a/seedu/address/logic/parser/AddCommandParser.java	80.00% <0.00%> (-12.86%)	4.00 <0.00> (ø)
.../seedu/address/logic/parser/CommonCentsParser.java	48.00% <0.00%> (-2.00%)	8.00 <0.00> (ø)
.../seedu/address/logic/parser/FindCommandParser.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
... and 10 more

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