Ensure unique matric number and room

[athuyaoo]

If you enter a duplicate matric number or room, there'll be an error.

Closes #199, #195

[schoolex]

LGTM
You can merge it after resolving the conflicts

[codecov-io]

Codecov Report

Merging #226 into master will decrease coverage by 0.09%.
The diff coverage is 52.38%.

@@             Coverage Diff              @@
##             master     #226      +/-   ##
============================================
- Coverage     67.69%   67.60%   -0.10%     
- Complexity      666      674       +8     
============================================
  Files           110      110              
  Lines          2294     2315      +21     
  Branches        282      289       +7     
============================================
+ Hits           1553     1565      +12     
- Misses          607      612       +5     
- Partials        134      138       +4     

Impacted Files	Coverage Δ	Complexity Δ
.../java/seedu/address/logic/commands/AddCommand.java	75.00% <0.00%> (-25.00%)	8.00 <0.00> (ø)
src/main/java/seedu/address/model/Model.java	100.00% <ø> (ø)	1.00 <0.00> (ø)
...java/seedu/address/logic/commands/EditCommand.java	90.81% <28.57%> (-4.79%)	14.00 <0.00> (+2.00)	⬇️
...rc/main/java/seedu/address/model/ModelManager.java	94.28% <90.00%> (-0.72%)	33.00 <5.00> (+5.00)	⬇️
src/main/java/seedu/address/model/person/Room.java	61.29% <0.00%> (+3.22%)	15.00% <0.00%> (+1.00%)

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