Remove Tag class and make Role class standalone from Tag

[plosslaw]

Resolves #106

[codecov-io]

Codecov Report

Merging #129 into master will decrease coverage by 0.05%.
The diff coverage is 72.00%.

@@             Coverage Diff              @@
##             master     #129      +/-   ##
============================================
- Coverage     69.82%   69.77%   -0.06%     
+ Complexity      863      854       -9     
============================================
  Files           139      137       -2     
  Lines          2993     2971      -22     
  Branches        411      408       -3     
============================================
- Hits           2090     2073      -17     
+ Misses          778      775       -3     
+ Partials        125      123       -2     

Impacted Files	Coverage Δ	Complexity Δ
...java/mcscheduler/logic/commands/AssignCommand.java	90.38% <ø> (ø)	14.00 <0.00> (ø)
...mcscheduler/logic/commands/CancelLeaveCommand.java	89.74% <ø> (ø)	12.00 <0.00> (ø)
...n/java/mcscheduler/logic/commands/CommandUtil.java	88.46% <ø> (ø)	12.00 <0.00> (ø)
...scheduler/logic/commands/MassTakeLeaveCommand.java	95.74% <ø> (ø)	17.00 <0.00> (ø)
...va/mcscheduler/logic/commands/ReassignCommand.java	77.58% <ø> (ø)	15.00 <0.00> (ø)
...ava/mcscheduler/logic/commands/RoleAddCommand.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
.../mcscheduler/logic/commands/RoleDeleteCommand.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...va/mcscheduler/logic/commands/RoleListCommand.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...a/mcscheduler/logic/commands/ShiftEditCommand.java	62.50% <ø> (ø)	15.00 <0.00> (ø)
...a/mcscheduler/logic/commands/TakeLeaveCommand.java	100.00% <ø> (ø)	9.00 <0.00> (ø)
... and 31 more

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[WangZijun97]

LGTM! In future can consider adding tests to ParserUtil for all our various added parser methods.