Undo/Redo implementation

[YangYue128-helen]

No description provided.

[JinHao-L]

Other than the minor comments, the rest LGTM 👍
The comments can be addressed in a separate PR if necessary

[xz0127]

Other than the minor comments mentioned, the rest LGTM! 👍 Good work and perhaps some additional parts have to be added when change and available commands are added in.

[codecov-io]

Codecov Report

Merging #125 into master will increase coverage by 0.79%.
The diff coverage is 92.95%.

@@             Coverage Diff              @@
##             master     #125      +/-   ##
============================================
+ Coverage     77.17%   77.96%   +0.79%     
- Complexity      800      848      +48     
============================================
  Files           111      117       +6     
  Lines          2331     2451     +120     
  Branches        276      286      +10     
============================================
+ Hits           1799     1911     +112     
- Misses          451      455       +4     
- Partials         81       85       +4     

Impacted Files	Coverage Δ	Complexity Δ
.../java/seedu/address/logic/parser/NuudleParser.java	64.00% <0.00%> (-5.57%)	12.00 <0.00> (ø)
src/main/java/seedu/address/model/Model.java	100.00% <ø> (ø)	2.00 <0.00> (ø)
...java/seedu/address/logic/commands/RedoCommand.java	81.81% <81.81%> (ø)	3.00 <3.00> (?)
...java/seedu/address/logic/commands/UndoCommand.java	81.81% <81.81%> (ø)	3.00 <3.00> (?)
...rc/main/java/seedu/address/model/ModelManager.java	96.07% <94.73%> (+0.72%)	49.00 <19.00> (+10.00)
.../seedu/address/model/VersionedAppointmentBook.java	96.66% <96.66%> (ø)	15.00 <15.00> (?)
...java/seedu/address/model/VersionedPatientBook.java	96.66% <96.66%> (ø)	15.00 <15.00> (?)
.../java/seedu/address/logic/commands/AddCommand.java	100.00% <100.00%> (ø)	8.00 <0.00> (ø)
...va/seedu/address/logic/commands/AssignCommand.java	88.46% <100.00%> (+0.46%)	11.00 <0.00> (ø)
...va/seedu/address/logic/commands/CancelCommand.java	100.00% <100.00%> (ø)	10.00 <0.00> (ø)
... and 13 more

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