Add role-related commands and checks for valid roles

[wakululuu]

Resolves #81

[codecov-io]

Codecov Report

Merging #102 into master will decrease coverage by 3.01%.
The diff coverage is 36.90%.

@@             Coverage Diff              @@
##             master     #102      +/-   ##
============================================
- Coverage     68.03%   65.02%   -3.02%     
- Complexity      651      666      +15     
============================================
  Files           117      125       +8     
  Lines          2212     2410     +198     
  Branches        258      283      +25     
============================================
+ Hits           1505     1567      +62     
- Misses          629      754     +125     
- Partials         78       89      +11     

Impacted Files	Coverage Δ	Complexity Δ
...main/java/seedu/address/commons/core/Messages.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...va/seedu/address/logic/commands/AssignCommand.java	92.30% <ø> (-0.38%)	14.00 <0.00> (ø)
...a/seedu/address/logic/commands/RoleAddCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...eedu/address/logic/commands/RoleDeleteCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
.../seedu/address/logic/commands/RoleListCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...du/address/logic/commands/WorkerDeleteCommand.java	81.81% <ø> (ø)	8.00 <0.00> (ø)
...edu/address/logic/parser/RoleAddCommandParser.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
.../address/logic/parser/RoleDeleteCommandParser.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...du/address/logic/parser/ShiftAddCommandParser.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...u/address/logic/parser/ShiftEditCommandParser.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
... and 34 more

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[tnsyn]

Small comment but I think we don't have to throw this, since AssignmentNotFoundException is a runtime exception? Also a bit inconsistent with other methods like deleteWorker/deleteShift which do not throw WorkerNotFoundException/ShiftNotFoundException respectively

[wakululuu]

It seems that it is a practice to make RuntimeExceptions checked exceptions if the client is able to recover from them (https://docs.oracle.com/javase/tutorial/essential/exceptions/runtime.html). But I do agree it is inconsistent with the other methods so I have changed the implementation of UnassignCommand#execute() itself.

[tnsyn]

Same small issue!

[tnsyn]

LGTM